Optical stack for switchable directional display

ABSTRACT

A privacy display comprises a spatial light modulator and a compensated switchable guest-host liquid crystal retarder arranged between first and second polarisers arranged in series with the spatial light modulator. In a privacy mode of operation, on-axis light from the spatial light modulator is directed without loss, whereas off-axis light has reduced luminance. The visibility of the display to off-axis snoopers is reduced by means of luminance reduction over a wide polar field. In a wide angle mode of operation, the switchable liquid crystal retardance is adjusted so that off-axis luminance is substantially unmodified.

TECHNICAL FIELD

This disclosure generally relates to illumination from light modulationdevices, and more specifically relates to switchable optical stacks forproviding control of illumination for use in a display including aprivacy display.

BACKGROUND

Privacy displays provide image visibility to a primary user that istypically in an on-axis position and reduced visibility of image contentto a snooper, that is typically in an off-axis position. A privacyfunction may be provided by micro-louvre optical films that transmitsome light from a display in an on-axis direction with low luminance inoff-axis positions. However such films have high losses for head-onillumination and the micro-louvres may cause Moiré artefacts due tobeating with the pixels of the spatial light modulator. The pitch of themicro-louvre may need selection for panel resolution, increasinginventory and cost.

Switchable privacy displays may be provided by control of the off-axisoptical output.

Control may be provided by means of luminance reduction, for example bymeans of switchable backlights for a liquid crystal display (LCD)spatial light modulator. Display backlights in general employ waveguidesand edge emitting sources. Certain imaging directional backlights havethe additional capability of directing the illumination through adisplay panel into viewing windows. An imaging system may be formedbetween multiple sources and the respective window images. One exampleof an imaging directional backlight is an optical valve that may employa folded optical system and hence may also be an example of a foldedimaging directional backlight. Light may propagate substantially withoutloss in one direction through the optical valve whilecounter-propagating light may be extracted by reflection off tiltedfacets as described in U.S. Pat. No. 9,519,153, which is hereinincorporated by reference in its entirety.

Control of off-axis privacy may further be provided by means of contrastreduction, for example by adjusting the liquid crystal bias tilt in anIn-Plane-Switching LCD.

BRIEF SUMMARY

According to a first aspect of the present disclosure there is provideda display device comprising: a spatial light modulator comprising alayer of addressable pixels; a display polariser arranged on a side ofthe spatial light modulator; the display polariser being a linearpolariser; and a guest-host liquid crystal retarder comprising a liquidcrystal layer comprising a guest material and a host material; whereinthe guest material is an anisotropic material and the host material is aliquid crystal material; the gust-host liquid crystal retarder beingarranged on the same side of the spatial light modulator as the displaypolariser with the display polariser arranged between the guest-hostliquid crystal retarder and the spatial light modulator; wherein theoptical axis of the guest-host liquid crystal retarder has an alignmentcomponent perpendicular to the plane of the guest-host liquid crystalretarder in at least a state of the host material. Advantageously adisplay may be provided with a privacy function.

The anisotropic material may be an anisotropic absorber. The anisotropicabsorber may be a dichroic dye or a pleochroic dye and the anisotropicmaterial may be a dichroic dye or a pleochroic dye. Advantageously a lowstray light display may be provided, and off-axis luminance may bereduced for a privacy display.

The anisotropic material may comprise metallic nanomaterial such assilver nanowires. The metallic nanomaterial may comprise a transparentelectrically insulating surface layer. The insulating surface layer maybe coating or may be formed chemically such as a transparent oxide forexample. Advantageously off-axis ambient light may be reflected toprovide reduced image contrast to an off-axis observer and increase theprivacy effect. In a backlight, off-axis light may be recirculated intothe backlight, increasing efficiency.

The guest material may comprise less than 3%, preferably less than 2%and most preferably less than 1% of the host material by volume.Alternatively, the guest material may comprise less than 3%, preferablyless than 2% and most preferably less than 1% of the host material byweight. Advantageously when the guest material is a solid the relativeproportions may be more conveniently measured. The guest material maycomprise a positive dichroic dye material or a positive pleochroic dyematerial and the optical axis of the guest-host liquid crystal layer hasan alignment component in the plane of the guest-host liquid crystallayer that is orthogonal to the electric vector transmission directionof the display polariser. The on-axis extinction coefficient of theguest-host liquid retarder in at least one state of the host material ina mode of operation may be at least 60%, preferably at least 80% andmost preferably at least 90%.

The display device may further comprise at least one passive retarderarranged between the display polariser and the guest-host liquid crystalretarder. Advantageously the field of view of a privacy display may bereduced.

The guest material may comprise liquid crystal material that is cured.Advantageously cost and thickness may be reduced.

The display device may further comprise an additional polariser; that isa linear polariser and is arranged on the same side of the spatial lightmodulator as the display polariser with the guest-host liquid crystalretarder arranged between the display polariser and the additionalpolariser. The display polariser and the additional polariser may haveelectric vector transmission directions that are parallel. The displaydevice may further comprise at least one passive retarder arrangedbetween the guest-host liquid crystal layer and the additionalpolariser. Advantageously the field of view of a privacy display may bereduced.

The guest-host liquid crystal retarder may comprise a switchable liquidcrystal retarder further comprising transparent electrodes arranged toapply a voltage capable of switching host material between at least twostates, in one of which states the optical axis of the guest-host liquidcrystal retarder has an alignment component perpendicular to the planeof the guest-host liquid crystal retarder. The electrodes may be onopposite sides of the layer of liquid crystal layer. The display devicemay further comprise a control system arranged to control the voltageapplied across the electrodes of the at least one switchable liquidcrystal retarder.

The switchable liquid crystal retarder may comprise two surfacealignment layers disposed adjacent to the layer liquid crystal materialand on opposite sides thereof and each arranged to provide homeotropicalignment in the adjacent liquid crystal material. By the application ofan electric field, advantageously a display may be switched between alow stray light display mode such as a privacy mode to a wide angle modefor multiple display users and increased image uniformity. In that case,the following features may be present.

The host material may be a liquid crystal material with a negativedielectric anisotropy.

The liquid crystal layer may have a retardance for light of a wavelengthof 550 nm in a range from 500 nm to 1000 nm, preferably in a range from600 nm to 900 nm and most preferably in a range from 700 nm to 850 nm.

The at least one passive retarder may comprise a retarder having itsoptical axis perpendicular to the plane of the retarder, the at leastone passive retarder having a retardance for light of a wavelength of550 nm in a range from −300 nm to −900 nm, preferably in a range from−450 nm to −800 nm and most preferably in a range from −500 nm to −725nm. Alternatively, the at least one passive retarder may comprise a pairof retarders which have optical axes in the plane of the retarders thatare crossed, each retarder of the pair of retarders having a retardancefor light of a wavelength of 550 nm in a range from 300 nm to 800 nm,preferably in a range from 500 nm to 700 nm and most preferably in arange from 550 nm to 675 nm. Advantageously low voltage operation may beprovided in a wide angle mode of operation, reducing power consumption.

The switchable liquid crystal retarder may comprise two surfacealignment layers disposed adjacent to the layer of liquid crystalmaterial and on opposite sides thereof and each arranged to providehomogeneous alignment in the adjacent liquid crystal material. In thatcase, the following features may be present.

The layer of liquid crystal material of the switchable liquid crystalretarder may comprise a liquid crystal material with a positivedielectric anisotropy.

The layer of liquid crystal material may have a retardance for light ofa wavelength of 550 nm in a range from 500 nm to 1000 nm, preferably ina range from 600 nm to 850 nm and most preferably in a range from 700 nmto 800 nm.

The at least one passive compensation retarder may comprise a retarderhaving its optical axis perpendicular to the plane of the retarder, theat least one passive retarder having a retardance for light of awavelength of 550 nm in a range from −300 nm to −700 nm, preferably in arange from −350 nm to −600 nm and most preferably in a range from −400nm to −500 nm. Alternatively, the at least one passive compensationretarder may comprise a pair of retarders which have optical axes in theplane of the retarders that are crossed, each retarder of the pair ofretarders having a retardance for light of a wavelength of 550 nm in arange from 300 nm to 800 nm, preferably in a range from 350 nm to 650 nmand most preferably in a range from 450 nm to 550 nm. Advantageously thevisibility of material flow may be reduced.

The switchable liquid crystal retarder may comprise two surfacealignment layers disposed adjacent to the layer of liquid crystalmaterial and on opposite sides thereof, one of the surface alignmentlayers being arranged to provide homeotropic alignment in the adjacentliquid crystal material and the other of the surface alignment layersbeing arranged to provide homogeneous alignment in the adjacent liquidcrystal material. In that case, the following features may be present.

The surface alignment layer may be arranged to provide homogeneousalignment is between the layer of liquid crystal material and thecompensation retarder.

The layer of liquid crystal material may have a retardance for light ofa wavelength of 550 nm in a range from 700 nm to 2000 nm, preferably ina range from 1000 nm to 1500 nm and most preferably in a range from 1200nm to 1500 nm.

The at least one passive compensation retarder may comprise a retarderhaving its optical axis perpendicular to the plane of the retarder, theat least one passive retarder having a retardance for light of awavelength of 550 nm in a range from −400 nm to −1800 nm, preferably ina range from −700 nm to −1500 nm and most preferably in a range from−900 nm to −1300 nm. Alternatively, the at least one passivecompensation retarder may comprise a pair of retarders which haveoptical axes in the plane of the retarders that are crossed, eachretarder of the pair of retarders having a retardance for light of awavelength of 550 nm in a range from 400 nm to 1800 nm, preferably in arange from 700 nm to 1500 nm and most preferably in a range from 900 nmto 1300 nm.

Each alignment layer may have a pretilt having a pretilt direction witha component in the plane of the liquid crystal layer that is parallel oranti-parallel or orthogonal to the electric vector transmissiondirection of the display polariser. The display device may furthercomprise: an additional polariser arranged on the same side of thespatial light modulator as the display polariser; and plural retardersarranged between the additional polariser and the display polariser,wherein the plural retarders comprise: a switchable liquid crystalretarder comprising a layer of liquid crystal material; and at least onepassive compensation retarder.

The at least one passive retarder may be arranged to introduce no phaseshift to polarisation components of light passed by the one of thedisplay polariser and the additional polariser on the input side of theplural retarders along an axis along a normal to the plane of the atleast one passive compensation retarder. The at least one passiveretarder may be arranged to introduce a phase shift to polarisationcomponents of light passed by the one of the display polariser and theadditional polariser on the input side of the plural retarders along anaxis inclined to a normal to the plane of the at least one passivecompensation retarder.

The guest-host liquid crystal retarder may be arranged to introduce nophase shift to polarisation components of light passed by the one of thedisplay polariser and the additional polariser on the input side of theplural retarders along an axis along a normal to the plane of theguest-host liquid crystal retarder.

The guest-host liquid crystal retarder may be arranged to introduce aphase shift to polarisation components of light passed by the one of thedisplay polariser and the additional polariser on the input side of theplural retarders along an axis inclined to a normal to the plane of theswitchable liquid crystal retarder in at least a state of the guest-hostliquid crystal retarder.

The guest-host liquid crystal retarder may be arranged to not affect theluminance of light passing through the display polariser and theguest-host liquid crystal retarder along an axis along a normal to theplane of the guest-host liquid crystal retarder. The guest-host liquidcrystal retarder may be arranged to reduce the luminance of lightpassing through the display polariser and the guest-host liquid crystalretarder along an axis inclined to a normal to the plane of theretarders.

The display device may further comprise at least one further retarderand a further additional polariser, wherein the at least one furtherretarder is arranged between the first-mentioned additional polariserand the further additional polariser.

The display device may further comprise a backlight arranged to outputlight, wherein the spatial light modulator is a transmissive spatiallight modulator arranged to receive output light from the backlight.

The backlight may provide a luminance at polar angles to the normal tothe spatial light modulator greater than 45 degrees that is at most 33%of the luminance along the normal to the spatial light modulator,preferably at most 20% of the luminance along the normal to the spatiallight modulator, and most preferably at most 10% of the luminance alongthe normal to the spatial light modulator. Advantageously a lowluminance off-axis image may be seen in privacy mode.

The backlight may comprise: an array of light sources; a directionalwaveguide comprising: an input end extending in a lateral directionalong a side of the directional waveguide, the light sources beingdisposed along the input end and arranged to input input light into thewaveguide; and opposed first and second guide surfaces extending acrossthe directional waveguide from the input end for guiding light input atthe input end along the waveguide, the waveguide being arranged todeflect input light guided through the directional waveguide to exitthrough the first guide surface. The backlight may further comprise alight turning film and the directional waveguide is a collimatingwaveguide. The collimating waveguide may comprise (i) a plurality ofelongate lenticular elements; and (ii) a plurality of inclined lightextraction features, wherein the plurality of elongate lenticularelements and the plurality of inclined light extraction features areoriented to deflect input light guided through the directional waveguideto exit through the first guide surface.

The directional waveguide may be an imaging waveguide arranged to imagethe light sources in the lateral direction so that the output light fromthe light sources is directed into respective optical windows in outputdirections that are distributed in dependence on the input positions ofthe light sources.

The imaging waveguide may comprise a reflective end for reflecting theinput light back along the imaging waveguide, wherein the second guidesurface is arranged to deflect the reflected input light through thefirst guide surface as output light, the second guide surface compriseslight extraction features and intermediate regions between the lightextraction features, the light extraction features being oriented todeflect the reflected input light through the first guide surface asoutput light and the intermediate regions being arranged to direct lightthrough the waveguide without extracting it; and the reflective end haspositive optical power in the lateral direction extending between sidesof the waveguide that extend between the first and second guidesurfaces.

The display polariser may be an input display polariser arranged on theinput side of the spatial light modulator between the backlight and thespatial light modulator, and the guest-host liquid crystal retarder maybe arranged between the input display polariser and the backlight. Theadditional polariser may be a reflective polariser. The display devicemay further comprise an output polariser arranged on the output side ofthe spatial light modulator.

The display polariser may be an output polariser arranged on the outputside of the spatial light modulator. The display device may furthercomprise an input polariser arranged on the input side of the spatiallight modulator. The display device may further comprise a furtheradditional polariser arranged on the input side of the spatial lightmodulator and at least one further retarder arranged between the atleast one further additional polariser and the input polariser.

The spatial light modulator may comprise an emissive spatial lightmodulator arranged to output light and the display polariser may be anoutput display polariser arranged on the output side of the emissivespatial light modulator.

The display device may further comprise at least one further retarderand a further additional polariser, wherein the at least one furtherretarder may be arranged between the first-mentioned additionalpolariser and the further additional polariser.

The various features and alternatives set out above with respect to thefirst aspect of the present disclosure may similarly be applied to thesecond aspect of the present disclosure.

Embodiments of the present disclosure may be used in a variety ofoptical systems. The embodiments may include or work with a variety ofprojectors, projection systems, optical components, displays,microdisplays, computer systems, processors, self-contained projectorsystems, visual and/or audio-visual systems and electrical and/oroptical devices. Aspects of the present disclosure may be used withpractically any apparatus related to optical and electrical devices,optical systems, presentation systems or any apparatus that may containany type of optical system. Accordingly, embodiments of the presentdisclosure may be employed in optical systems, devices used in visualand/or optical presentations, visual peripherals and so on and in anumber of computing environments.

Before proceeding to the disclosed embodiments in detail, it should beunderstood that the disclosure is not limited in its application orcreation to the details of the particular arrangements shown, becausethe disclosure is capable of other embodiments. Moreover, aspects of thedisclosure may be set forth in different combinations and arrangementsto define embodiments unique in their own right. Also, the terminologyused herein is for the purpose of description and not of limitation.

These and other advantages and features of the present disclosure willbecome apparent to those of ordinary skill in the art upon reading thisdisclosure in its entirety.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are illustrated by way of example in the accompanyingFIGURES, in which like reference numbers indicate similar parts, and inwhich:

FIG. 1A is a schematic diagram illustrating in side perspective view anoptical stack of a directional display device comprising a frontswitchable guest-host liquid crystal retarder and an additionalpolariser;

FIG. 1B is a schematic diagram illustrating in front view alignment ofoptical layers in the optical stack of FIG. 1A;

FIG. 1C is a schematic diagram illustrating in side perspective view anoptical stack of a directional display device comprising an emissivespatial light modulator and a switchable guest-host liquid crystalretarder arranged on the output side of the emissive spatial lightmodulator;

FIG. 1D is a schematic diagram illustrating in side perspective view aview angle control optical element comprising a passive guest-hostliquid crystal retarder, a switchable guest-host liquid crystal retarderand a control polariser;

FIG. 1E is a schematic diagram illustrating in side perspective view anoptical stack of a directional display device comprising a frontswitchable guest-host liquid crystal retarder;

FIG. 1F is a schematic diagram illustrating in front view alignment ofoptical layers in the optical stack of FIG. 1E;

FIG. 2A is a schematic diagram illustrating in side perspective view anoptical stack of a directional display device comprising a backlight, arear switchable guest-host liquid crystal retarder, and a transmissivespatial light modulator;

FIG. 2B is a schematic diagram illustrating in side perspective view anoptical stack of a directional display device comprising a backlight, arear passive guest-host liquid crystal retarder, and a transmissivespatial light modulator;

FIG. 2C is a schematic diagram illustrating in side perspective view anoptical stack of a directional display device comprising a backlight, arear switchable guest-host liquid crystal retarder, and a transmissivespatial light modulator wherein the additional polariser comprises alinear absorbing polariser;

FIG. 3A is a schematic diagram illustrating in side perspective view aswitchable guest-host liquid crystal retarder in a privacy mode ofoperation;

FIG. 3B is a schematic graph illustrating the simulated variation ofoutput transmission with polar direction for the transmitted light raysin FIG. 3A;

FIG. 3C is a schematic graph illustrating the simulated variation ofoutput transmission with lateral angle at zero elevation with varyingconcentration of dichroic dye for the transmitted light rays in FIG. 3A;

FIG. 3D is a schematic graph illustrating the simulated variation ofnormalised output transmission with lateral angle at zero elevation withvarying concentration of dichroic dye for the transmitted light rays inFIG. 3A;

FIG. 3E is a schematic diagram illustrating in side perspective view aswitchable guest-host liquid crystal retarder in a wide angle mode ofoperation;

FIG. 3F is a schematic graph illustrating the variation of outputtransmission with polar direction for the transmitted light rays in FIG.3E;

FIG. 3G is a schematic diagram illustrating in side view a method toform a passive guest-host liquid crystal retarder;

FIG. 3H is a schematic diagram illustrating in side view a passiveguest-host liquid crystal retarder wherein the guest material comprisessilver nanowires;

FIG. 3I is a schematic diagram illustrating in perspective side view adisplay comprising a passive guest-host liquid crystal retarder whereinthe guest material comprises silver nanowires and an additionalpolariser;

FIG. 4A is a schematic diagram illustrating in side perspective view aswitchable guest-host liquid crystal retarder and an additionalpolariser in a privacy mode of operation;

FIG. 4B is a schematic graph illustrating the simulated variation ofoutput transmission with polar direction for the transmitted light raysin FIG. 4A with no guest dichroic dye material;

FIG. 4C is a schematic graph illustrating the simulated variation ofoutput transmission with polar direction for the transmitted light raysin FIG. 4A with a first concentration of guest dichroic dye material;

FIG. 4D is a schematic graph illustrating the simulated variation ofoutput transmission with polar direction for the transmitted light raysin FIG. 4A with a second concentration of guest dichroic dye material;

FIG. 4E is a schematic graph illustrating the simulated variation ofoutput transmission with lateral angle at zero elevation with varyingconcentration of dichroic dye for the transmitted light rays in FIG. 4A;

FIG. 4F is a schematic graph illustrating the simulated variation ofnormalised output transmission with lateral angle at zero elevation withvarying concentration of dichroic dye for the transmitted light rays inFIG. 4A;

FIG. 4G is a schematic diagram illustrating in perspective side view anarrangement of a switchable compensated retarder comprising a negativeC-plate in a wide angle mode of operation;

FIG. 4H is a schematic diagram illustrating a graph of liquid crystaldirector angle against fractional location through the switchable liquidcrystal retarder cell;

FIG. 4I is a schematic diagram illustrating in side view propagation ofoutput light from a spatial light modulator through the optical stack ofFIG. 4G in a wide angle mode of operation;

FIG. 4J is a schematic graph illustrating the variation of outputtransmission with polar direction for the transmitted light rays in FIG.4I;

FIG. 5A is a schematic diagram illustrating in perspective side view anarrangement of a switchable compensated guest-host liquid crystalretarder comprising a negative C-plate in a privacy mode of operation;

FIG. 5B is a schematic diagram illustrating in side view propagation ofoutput light from a spatial light modulator through the optical stack ofFIG. 5A in a privacy mode of operation;

FIG. 5C is a schematic graph illustrating the simulated variation ofoutput transmission with polar direction for the transmitted light raysin FIG. 5B with no guest dichroic dye material;

FIG. 5D is a schematic graph illustrating the simulated variation ofoutput transmission with polar direction for the transmitted light raysin FIG. 5B with a first concentration of guest dichroic dye material;

FIG. 5E is a schematic graph illustrating the simulated variation ofoutput transmission with polar direction for the transmitted light raysin FIG. 5B with a second concentration of guest dichroic dye material;

FIG. 6A is a schematic diagram illustrating in front perspective viewobservation of transmitted output light for a display operating inprivacy mode;

FIG. 6B is a schematic diagram illustrating in front perspective viewsthe appearance of the display of FIGS. 1A-1C operating in privacy mode;

FIG. 6C is a schematic diagram illustrating in side view an automotivevehicle with a switchable directional display arranged within thevehicle cabin for both entertainment and sharing modes of operation;

FIG. 6D is a schematic diagram illustrating in top view an automotivevehicle with a switchable directional display arranged within thevehicle cabin in an entertainment mode of operation;

FIG. 6E is a schematic diagram illustrating in top view an automotivevehicle with a switchable directional display arranged within thevehicle cabin in a sharing mode of operation;

FIG. 6F is a schematic diagram illustrating in top view an automotivevehicle with a switchable directional display arranged within thevehicle cabin for both night-time and day-time modes of operation;

FIG. 6G is a schematic diagram illustrating in side view an automotivevehicle with a switchable directional display arranged within thevehicle cabin in a night-time mode of operation;

FIG. 6H is a schematic diagram illustrating in side view an automotivevehicle with a switchable directional display arranged within thevehicle cabin in a day-time mode of operation;

FIG. 7A, FIG. 7B, FIG. 7C, and FIG. 7D are schematic diagramsillustrating the variation of output transmission with polar directionfor different drive voltages;

FIG. 8A is a schematic diagram illustrating in perspective side view anarrangement of a switchable compensated retarder in a wide angle mode ofoperation comprising crossed A-plate passive compensation retarders andhomeotropically aligned switchable liquid crystal retarder;

FIG. 8B is a schematic diagram illustrating in perspective side view anarrangement of a switchable compensated retarder in a privacy mode ofoperation comprising crossed A-plate passive compensation retarders andhomeotropically aligned switchable liquid crystal retarder;

FIG. 9A is a schematic graph illustrating the variation of outputtransmission with polar direction for transmitted light rays in FIG. 8Ain a wide angle mode of operation;

FIG. 9B is a schematic graph illustrating the variation of outputtransmission with polar direction for transmitted light rays in FIG. 8Bin a privacy mode of operation;

FIG. 10A and FIG. 10B are schematic diagrams illustrating in perspectiveside view an arrangement of a switchable compensated retarder in a wideangle mode and a privacy mode of operation respectively comprising ahomogeneously aligned switchable liquid crystal retarder and a passivenegative C-plate retarder;

FIG. 10C is a schematic diagram illustrating a graph of liquid crystaldirector angle against fractional location through the switchable liquidcrystal retarder cell of FIG. 10A for different applied voltages;

FIG. 11A, FIG. 11B, and FIG. 11C are schematic graphs illustrating thevariation of output transmission with polar direction for transmittedlight rays of switchable compensated retarder comprising a homogeneouslyaligned liquid crystal cell and a negative C-plate in a privacy mode andfor two different wide angle mode addressing drive voltagesrespectively;

FIG. 12 is a schematic diagram illustrating in perspective side view anarrangement of a switchable compensated retarder in a privacy mode ofoperation comprising crossed A-plate passive compensation retarders andhomogeneously aligned switchable liquid crystal retarder;

FIG. 13, FIG. 14, and FIG. 15 are schematic graphs illustrating thevariation of output transmission with polar direction for transmittedlight rays of switchable compensated retarder comprising a homogeneouslyaligned liquid crystal cell and crossed A-plates in a privacy mode andwide angle modes for different drive voltages;

FIG. 16A and FIG. 16B are schematic diagrams illustrating in side viewspart of a display comprising a switchable compensated retarder andoptical bonding layers;

FIG. 17 is a schematic diagram illustrating in perspective side view anarrangement of a switchable compensated retarder in a privacy mode ofoperation comprising crossed A-plate passive compensation retarders andhomogeneously aligned switchable liquid crystal retarder, furthercomprising a passive rotation retarder;

FIG. 18 is a schematic diagram illustrating in perspective side view anarrangement of a switchable compensated retarder in a privacy mode ofoperation comprising a homogeneously and homeotropically alignedswitchable liquid crystal retarder and a passive negative C-plateretarder;

FIG. 19 is a schematic graph illustrating the variation of outputtransmission with polar direction for transmitted light rays in FIG. 18in a privacy mode of operation;

FIG. 20 is a schematic graph illustrating the variation of outputtransmission with polar direction for transmitted light rays in FIG. 18in a wide angle mode of operation;

FIG. 21 is a schematic diagram illustrating in perspective side view anarrangement of a switchable compensated retarder in a privacy mode ofoperation comprising a negative C-plate passive compensation retarderand homeotropically aligned switchable liquid crystal retarder arrangedbetween the output polariser and additional polariser; and a negativeC-plate passive compensation retarder and homeotropically alignedswitchable liquid crystal retarder arranged between the first-mentionedadditional polariser and further additional polariser in a privacy modeof operation;

FIG. 22 is a schematic diagram illustrating in side perspective view aview angle control optical element comprising a first passivecompensation retarder, a first switchable liquid crystal retarder, afirst control polariser, a second passive compensation retarder, asecond switchable liquid crystal retarder and a second controlpolariser;

FIG. 23A is a schematic diagram illustrating in top view an automotivevehicle with a switchable directional display arranged within thevehicle cabin for day-time and/or sharing modes of operation;

FIG. 23B is a schematic diagram illustrating in side view an automotivevehicle with a switchable directional display arranged within thevehicle cabin for day-time and/or sharing modes of operation;

FIG. 24A is a schematic diagram illustrating in top view an automotivevehicle with a switchable directional display arranged within thevehicle cabin for night-time and/or entertainment modes of operation;

FIG. 24B is a schematic diagram illustrating in side view an automotivevehicle with a switchable directional display arranged within thevehicle cabin for night-time and/or entertainment modes of operation;

FIG. 25A is a schematic diagram illustrating in front perspective view adirectional backlight;

FIG. 25B is a schematic diagram illustrating in front perspective view anon-directional backlight;

FIG. 26 is a schematic graph illustrating variation with luminance withlateral viewing angle of displays with different fields of view;

FIG. 27A is a schematic diagram illustrating in side view a switchabledirectional display apparatus comprising an imaging waveguide andswitchable liquid crystal retarder;

FIG. 27B is a schematic diagram illustrating in rear perspective viewoperation of an imaging waveguide in a narrow angle mode of operation;

FIG. 27C is a schematic graph illustrating a field-of-view luminanceplot of the output of

FIG. 27B when used in a display apparatus with no switchable liquidcrystal retarder;

FIG. 28A is a schematic diagram illustrating in side view a switchabledirectional display apparatus comprising a switchable collimatingwaveguide and a switchable liquid crystal retarder operating in aprivacy mode of operation;

FIG. 28B is a schematic diagram illustrating in top view output of acollimating waveguide;

FIG. 28C is a schematic graph illustrating an iso-luminancefield-of-view polar plot for the display apparatus of FIG. 28A;

FIG. 29A is a schematic diagram illustrating in perspective viewillumination of a retarder layer by off-axis light;

FIG. 29B is a schematic diagram illustrating in perspective viewillumination of a retarder layer by off-axis light of a first linearpolarization state at 0 degrees;

FIG. 29C is a schematic diagram illustrating in perspective viewillumination of a retarder layer by off-axis light of a first linearpolarization state at 90 degrees;

FIG. 29D is a schematic diagram illustrating in perspective viewillumination of a retarder layer by off-axis light of a first linearpolarization state at 45 degrees;

FIG. 30A is a schematic diagram illustrating in perspective viewillumination of a C-plate retarder by off-axis polarised light with apositive elevation;

FIG. 30B is a schematic diagram illustrating in perspective viewillumination of a C-plate retarder by off-axis polarised light with anegative lateral angle;

FIG. 30C is a schematic diagram illustrating in perspective viewillumination of a C-plate retarder by off-axis polarised light with apositive elevation and negative lateral angle;

FIG. 30D is a schematic diagram illustrating in perspective viewillumination of a C-plate retarder by off-axis polarised light with apositive elevation and positive lateral angle;

FIG. 30E is a schematic graph illustrating the variation of outputtransmission with polar direction for transmitted light rays in FIGS.30A-D;

FIG. 31A is a schematic diagram illustrating in perspective viewillumination of crossed A-plate retarder layers by off-axis polarisedlight with a positive elevation;

FIG. 31B is a schematic diagram illustrating in perspective viewillumination of crossed A-plate retarder layers by off-axis polarisedlight with a negative lateral angle;

FIG. 31C is a schematic diagram illustrating in perspective viewillumination of crossed A-plate retarder layers by off-axis polarisedlight with a positive elevation and negative lateral angle;

FIG. 31D is a schematic diagram illustrating in perspective viewillumination of crossed A-plate retarder layers by off-axis polarisedlight with a positive elevation and positive lateral angle; and

FIG. 31E is a schematic graph illustrating the variation of outputtransmission with polar direction for transmitted light rays in FIGS.31A-D.

DETAILED DESCRIPTION

Terms related to optical retarders for the purposes of the presentdisclosure will now be described.

In a layer comprising a uniaxial birefringent material there is adirection governing the optical anisotropy whereas all directionsperpendicular to it (or at a given angle to it) are opticallyequivalent.

Optical axis refers to the direction of propagation of an unpolarisedlight ray in the uniaxial birefringent material in which nobirefringence is experienced by the ray. For light propagating in adirection orthogonal to the optical axis, the optical axis is the slowaxis when linearly polarized light with an electric vector directionparallel to the slow axis travels at the slowest speed. The slow axisdirection is the direction with the highest refractive index at thedesign wavelength. Similarly the fast axis direction is the directionwith the lowest refractive index at the design wavelength.

For positive optical anisotropy uniaxial birefringent materials the slowaxis direction is the extraordinary axis of the birefringent material.For negative optical anisotropy uniaxial birefringent materials the fastaxis direction is the extraordinary axis of the birefringent material.

The terms half a wavelength and quarter a wavelength refer to theoperation of a retarder for a design wavelength λ₀ that may typically bebetween 450 nm and 570 nm. In the present illustrative embodimentsexemplary retardance values are provided for a wavelength of 550 nmunless otherwise specified.

The retarder provides a phase shift between two perpendicularpolarization components of the light wave incident thereon and ischaracterized by the amount of relative phase, Γ, that it imparts on thetwo polarization components; which is related to the birefringence Δnand the thickness d of the retarder by

Γ=2·π·Δn·d/λ ₀  eqn. 1

In eqn. 1, Δn is defined as the difference between the extraordinary andthe ordinary index of refraction, i.e.

Δn=n _(e) −n _(o)  eqn. 2

For a half wave retarder, the relationship between d, Δn, and λ₀ ischosen so that the phase shift between polarization components is Γ=π.For a quarter wave retarder, the relationship between d, Δn, and λ₀ ischosen so that the phase shift between polarization components is Γ=π/2.

The term half wave retarder herein typically refers to light propagatingnormal to the retarder and normal to the spatial light modulator.

In the present disclosure an ‘A-plate’ refers to an optical retarderutilizing a layer of birefringent material with its optical axisparallel to the plane of the layer. The plane of the retarders refers tothe slow axis of the retarders extend in a plane, that is the x-y plane.

A ‘positive A-plate’ refers to positively birefringent A-plates, i.e.A-plates with a positive Δn.

In the present disclosure a ‘C-plate’ refers to an optical retarderutilizing a layer of birefringent material with its optical axisperpendicular to the plane of the layer. A ‘positive C-plate’ refers topositively birefringent C-plate, i.e. a C-plate with a positive Δn. A‘negative C-plate’ refers to a negatively birefringent C-plate, i.e. aC-plate with a negative Δn.

In the present disclosure an ‘O-plate’ refers to an optical retarderutilizing a layer of birefringent material with its optical axis havinga component parallel to the plane of the layer and a componentperpendicular to the plane of the layer. A ‘positive O-plate’ refers topositively birefringent O-plates, i.e. O-plates with a positive Δn.

Achromatic retarders may be provided wherein the material of theretarder is provided with an retardance Δn·d that varies with wavelengthλ as

Δn·d/λ=κ  eqn. 3

where κ is substantially a constant.

Examples of suitable materials include modified polycarbonates fromTeijin Films. Achromatic retarders may be provided in the presentembodiments to advantageously minimise colour changes between polarangular viewing directions which have low luminance reduction and polarangular viewing directions which have increased luminance reductions aswill be described below.

Various other terms used in the present disclosure related to retardersand to liquid crystals will now be described.

A liquid crystal cell has a retardance given by Δn·d where Δn is thebirefringence of the liquid crystal material in the liquid crystal celland d is the thickness of the liquid crystal cell, independent of thealignment of the liquid crystal material in the liquid crystal cell.

Homogeneous alignment refers to the alignment of liquid crystals inswitchable liquid crystal displays where molecules align substantiallyparallel to a substrate. Homogeneous alignment is sometimes referred toas planar alignment. Homogeneous alignment may typically be providedwith a small pre-tilt such as 2 degrees, so that the molecules at thesurfaces of the alignment layers of the liquid crystal cell are slightlyinclined as will be described below. Pretilt is arranged to minimisedegeneracies in switching of cells.

In the present disclosure, in the context of liquid crystal orientation,state refers to the orientation of the liquid crystal director at aparticular applied voltage i.e. a voltage applied state. The state maybe the zero voltage applied state.

In the present disclosure, homeotropic alignment is the state in whichrod-like liquid crystalline molecules align substantiallyperpendicularly to the substrate. In discotic liquid crystalshomeotropic alignment is defined as the state in which an axis of thecolumn structure, which is formed by disc-like liquid crystallinemolecules, aligns perpendicularly to a surface. In homeotropicalignment, pretilt is the tilt angle of the molecules that are close tothe alignment layer and is typically close to 90 degrees and for examplemay be 88 degrees.

Liquid crystal molecules with positive dielectric anisotropy areswitched from a homogeneous alignment (such as an A-plate retarderorientation) to a homeotropic alignment (such as a C-plate or O-plateretarder orientation) by means of an applied electric field.

Liquid crystal molecules with negative dielectric anisotropy areswitched from a homeotropic alignment (such as a C-plate or O-plateretarder orientation) to a homogeneous alignment (such as an A-plateretarder orientation) by means of an applied electric field.

Rod-like molecules have a positive birefringence so that n_(e)>n_(o) asdescribed in eqn. 2. Discotic molecules have negative birefringence sothat n_(e)<n_(o).

Positive retarders such as A-plates, positive O-plates and positiveC-plates may typically be provided by stretched films or rod-like liquidcrystal molecules. Negative retarders such as negative C-plates may beprovided by stretched films or discotic like liquid crystal molecules.

Parallel liquid crystal cell alignment refers to the alignment directionof homogeneous alignment layers being parallel or more typicallyantiparallel. In the case of pre-tilted homeotropic alignment, thealignment layers may have components that are substantially parallel orantiparallel. Hybrid aligned liquid crystal cells may have onehomogeneous alignment layer and one homeotropic alignment layer. Twistedliquid crystal cells may be provided by alignment layers that do nothave parallel alignment, for example oriented at 90 degrees to eachother.

A dichroic material has different absorption coefficients for lightpolarized in different directions. A pleochroic material absorbsdifferent wavelengths of light differently depending on the direction ofincidence of the rays or their plane of polarization, often resulting inthe appearance of different colours according to the direction of view.

Guest-host liquid crystal materials comprise a liquid crystal hostmaterial and a guest material comprising an anisotropic absorbing dye.The liquid crystal host material has a director direction thatrepresents the direction of the preferred orientation of molecules inthe neighbourhood of any point. By way of comparison with the presentembodiments, in the standard type of guest host display, linearlypolarized input light is absorbed by the dye molecules which arehomogeneously aligned with the polarizer transmission directionresulting in a black display state to the head-on viewer. When anelectric field is applied to the guest host liquid crystal cell, theliquid crystal host re-orientation causes the dye guest to reorient withit so that it is parallel to the applied electric field and so the inputpolarized light passes substantially without attenuation resulting in awhite display state to the head-on viewer. In both cases the off-axisviewer sees the same display state as the head-on viewer.

This is completely different from the guest host configuration used inthis specification where the head-on display brightness state of theguest host system is substantially unchanged by the application of theelectric field and only the off-axis viewing properties are altered.

When the liquid crystal host re-orients as described above there is achange in the retardance imparted to input light that arises from theoptical anisotropy of the liquid crystal molecules. In some (but notall) embodiments of a guest host system described herein the mainoperating effect is the re-orientation of the light absorbing dyemolecules and the retardance effect may be small so that only anegligible retardance is imparted to input light in the visiblewavelengths. This means liquid crystal host materials with low opticalanisotropy may be used.

In positive dichroic and pleochroic guest-host materials the majorabsorption axis of the dichroic material aligns with the liquid crystalhost director direction. In negative dichroic and pleochroic guest-hostmaterials the major absorption axis aligns perpendicular to the liquidcrystal host director direction.

The present description typically refer to positive dichroic dyematerials, however pleochroic and negative dichroic and pleochroicmaterials may also be used in the present embodiments as will be furtherdescribed.

The order parameter is used to describe the ordering of a liquid crystaland for liquid crystals in the nematic phase is typically less than 0.8,where an order parameter of 1 is for a perfectly aligned arrangement ofliquid crystal molecules and an order parameter of 0 is for an isotropicarrangement.

Transmissive spatial light modulators may further comprise retardersbetween the input display polariser and the output display polariser forexample as disclosed in U.S. Pat. No. 8,237,876, which is hereinincorporated by reference in its entirety. Such retarders (not shown)are in a different place to the passive retarders of the presentembodiments. Such retarders compensate for contrast degradations foroff-axis viewing locations, which is a different effect to the luminancereduction for off-axis viewing positions of the present embodiments.

Optical isolation retarders provided between the display polariser andan OLED display emission layer are described further in U.S. Pat. No.7,067,985. Optical isolation retarders are in a different place to thepassive retarders of the present embodiments. Isolation retarder reducesfrontal reflections of ambient light from the OLED display emissionlayer which is a different effect to the luminance reduction of emittedlight for off-axis viewing positions of the present embodiments.

The structure and operation of various switchable display devices willnow be described. In this description, common elements have commonreference numerals. It is noted that the disclosure relating to anyelement applies to each device in which the same or correspondingelement is provided. Accordingly, for brevity such disclosure is notrepeated.

FIG. 1A is a schematic diagram illustrating in side perspective view anoptical stack of a display device.

Display device 100 comprises a spatial light modulator 48 comprising atleast one display polariser, that is the output polariser 218. Theoutput polariser 218 is a linear polariser. Backlight 20 is arranged tooutput light and the spatial light modulator 48 comprises a transmissivespatial light modulator 48 arranged to receive output light from thebacklight 20. The display device 100 is arranged to output light 400with angular luminance properties as will be described herein.

In the present disclosure, the spatial light modulator 48 may comprise aliquid crystal display comprising substrates 212, 216, and liquidcrystal layer 214 having red, green and blue pixels 220, 222, 224. Thespatial light modulator 48 has an input display polariser 210 and anoutput display polariser 218 on opposite sides thereof. The outputdisplay polariser 218 is arranged to provide high extinction ratio forlight from the pixels 220, 222, 224 of the spatial light modulator 48.The input display polariser 210 and the output display polariser 218 areeach linear polarisers. Typical polarisers 210, 218 may be linearabsorbing polarisers such as stretched PVA iodine based polarisersbetween TAC layers.

Optionally a reflective polariser 208 may be provided between thedichroic input display polariser 210 and backlight 210 to providerecirculated light and increase display efficiency. Advantageouslyefficiency may be increased.

Backlight 20 may comprise input light sources 15, waveguide 1, rearreflector 3 and optical stack 5 comprising diffusers, light turningfilms and other known optical backlight structures. Asymmetricdiffusers, that may comprise asymmetric surface relief features forexample, may be provided in the optical stack 5 with increased diffusionin the elevation direction in comparison to the lateral direction may beprovided. Advantageously image uniformity may be increased.

In the present embodiments, the backlight 20 may be arranged to providean angular light distribution that has reduced luminance for off-axisviewing positions in comparison to head-on luminance as will bedescribed in FIGS. 26A to 28C below. Backlight 20 may further comprise aswitchable backlight arranged to switch the output angular luminanceprofile in order to provide reduced off-axis luminance in a privacy modeof operation and higher off-axis luminance in a wide angle mode ofoperation. Such switching backlight 20 may cooperate with the switchableguest-host retarder 300 of the present embodiments.

Additional polariser 318 is arranged on the same output side of thespatial light modulator 48 as the display output polariser 218. Theadditional polariser is a linear polariser and may be a linear absorbingpolariser.

The display polariser 218 and the additional polariser 318 have electricvector transmission directions 219, 319 that are parallel. As will bedescribed below, such parallel alignment provides high transmission forcentral viewing locations.

Plural retarders which together are referred to herein as a switchableguest-host retarder 300 are arranged between the additional polariser318 and the display polariser 218 and comprise: (i) a switchableguest-host liquid crystal retarder 301 comprising a liquid crystal layer314 of comprising a guest material and a host material arranged betweenthe display polariser 218 and the additional polariser 318; and (ii) apassive compensation retarder 330.

Thus at least one passive retarder 330 is arranged between the displaypolariser 218 and the guest-host liquid crystal retarder 301.

FIG. 1B is a schematic diagram illustrating in front view alignment ofoptical layers in the optical stack of FIG. 1A. The input electricvector transmission direction 211 at the input display polariser 210, ofthe spatial light modulator 48 provides an input polarisation componentthat may be transformed by the liquid crystal layer 214 to provideoutput polarisation component determined by the electric vectortransmission direction 219 of the output display polariser 218. Passivecompensation retarder 330 may comprise retardation layer with a discoticbirefringent material 430, while switchable guest-host liquid crystalretarder 301 may comprise liquid crystal material.

Switchable guest-host retarder 300 thus comprises a switchableguest-host liquid crystal retarder 301 comprising a switchableguest-host liquid crystal layer 314, substrates 312, 316 and passivecompensation retarder 330 arranged between and additional polariser 318and display polariser 218.

Substrates 312, 316 may be glass substrates or polymer substrates suchas polyimide substrates. Flexible substrates that may be convenientlyprovided with transparent electrodes may be provided. Advantageouslycurved, bent and foldable displays may be provided.

The display device 100 further comprises a control system 352 arrangedto control the voltage applied by voltage driver 350 across theelectrodes of the switchable guest-host liquid crystal retarder 301.

It may be desirable to provide reduced stray light or privacy control ofan emissive display.

FIG. 1C is a schematic diagram illustrating in side perspective view anoptical stack of a directional display device comprising an emissivespatial light modulator 48 and a switchable guest-host retarder 300arranged on the output side of the emissive spatial light modulator 48.

Spatial light modulator 48 may alternatively be provided by otherdisplay types that provide output light 400 by emission, such as organicLED displays (OLED), with output display polariser 218, substrates 512,516 and light emission layer 514. Output polariser 218 may providereduction of luminance for light reflected from the OLED pixel plane bymeans of one of more retarders 518 inserted between the output displaypolariser 218 and OLED pixel plane. The one or more retarders 518 may bea quarter waveplate and is different to the compensation retarder 330 ofthe present disclosure.

In the embodiment of FIG. 1C, the spatial light modulator 48 thuscomprises an emissive spatial light modulator and the display polariseris output display polariser 218.

Otherwise, the directional display device of FIG. 1C is the same as thatof FIG. 1A, as described above.

A view angle control optical element 260 for application to a displaydevice will now be described. View angle control optical elements 260may be added to spatial light modulators comprising a display polariser210, 218 to achieve switchable field-of-view characteristics.

FIG. 1D is a schematic diagram illustrating in side perspective view aview angle control optical element 260 for application to a displaydevice comprising a passive compensation retarder 330, a switchableguest-host liquid crystal retarder 301 and a control polariser 250.

In use, view angle control optical element 260 may be attached by a useror may be factory fitted to a polarised output spatial light modulator48. View angle control optical element 260 may be provided as a flexiblefilm for curved and bent displays. Alternatively the view angle controloptical element 260 may be provided on a rigid substrate such as a glasssubstrate.

Advantageously, an after-market privacy control element and/or straylight control element may be provided that does not require matching tothe panel pixel resolution to avoid Moiré artefacts. View angle controloptical element 260 may be further provided for factory fitting tospatial light modulator 48.

By attaching the view angle control optical element 260 of FIG. 1D to anexisting display device, it is possible to form a display device asshown in any of FIGS. 1A-C.

The embodiments of FIGS. 1A-D provide polar luminance control for light400 that is output from the spatial light modulator 48. That is, theswitchable guest-host retarder 300 (comprising the switchable guest-hostliquid crystal retarder 301 and the passive compensation retarder 330)does not affect the luminance of light passing through the input displaypolariser 210, the switchable guest-host retarder 300 and the additionalpolariser 318 along an axis along a normal to the plane of theswitchable guest-host retarder 300 but the switchable guest-hostretarder 300 does reduce the luminance of light passing therethroughalong an axis inclined to a normal to the plane of the switchableguest-host retarder 300, at least in one of the switchable states of thecompensated switchable retarder 300. The principles leading to thiseffect are described in greater detail below with reference to FIGS.29A-31E and arises from the presence or absence of a phase shiftintroduced by the switchable guest-host liquid crystal retarder 301 andthe passive compensation retarder 330 to light along axes that areangled differently with respect to the liquid crystal material of theswitchable guest-host liquid crystal retarder 301 and the passivecompensation retarder 330. A similar effect is achieved in all thedevices described below.

Furthermore, the provision of the passive compensation retarder 330 inaddition to the switchable guest-host liquid crystal retarder 301improves the performance, as will be described in more detail withreference to some specific display devices, and by comparison to somecomparative examples described with reference to FIGS. 19A-E.

It may be desirable to reduce the thickness and increase the efficiencyof the display apparatus.

FIG. 1E is a schematic diagram illustrating in side perspective view anoptical stack of a directional display device 100 comprising a frontswitchable guest-host liquid crystal retarder 301; and FIG. 1F is aschematic diagram illustrating in front view alignment of optical layersin the optical stack of FIG. 1E.

A display device comprises a spatial light modulator 48 comprising alayer 214 of addressable pixels 220, 222, 224; a display polariser 218arranged on the output side of the spatial light modulator 48; aguest-host liquid crystal retarder 301 comprising a liquid crystal layercomprising a guest material 414B and a host material 414A.

The display polariser 218 is arranged between the guest-host liquidcrystal retarder 301 and the layer 214 of addressable pixels.

In the embodiment of FIG. 1A, a passive compensation retarder 330 isarranged to provide control of incident polarisation state onto theguest-host liquid crystal retarder 301. By way of comparison FIG. 1Eillustrates that the retarder 330 may be omitted. Further in comparisonto the arrangement of FIG. 1A, additional polariser 318 is eliminated.Advantageously thickness and cost may be reduced.

It may be desirable to reduce the number of optical layers between aspatial light modulator 48 and an observer. An arrangement wherein theplural retarders 300 are arranged on the input side of the spatial lightmodulator 48 will now be described.

FIG. 2A is a schematic diagram illustrating in side perspective view anoptical stack of a directional display device comprising a backlight 20,a switchable rear guest-host retarder 301, a transmissive spatial lightmodulator 48 wherein the additional polariser 318 comprises a reflectivepolariser.

The display device 100 comprises a spatial light modulator 48; an inputdisplay polariser 210 arranged on the input side of the spatial lightmodulator 48. The switchable rear guest-host retarder 301 is alsoarranged on the input side of the spatial light modulator 48, the inputdisplay polariser 210 being between the switchable rear guest-hostretarder 300 and the spatial light modulator 48.

Additional polariser 318 is arranged on the same side of the spatiallight modulator 48 as the display polariser 210 with the guest-hostliquid crystal retarder 301 between the input display polariser 210 andthe additional polariser 318. Additional polariser 318 is a reflectivepolariser that operates in cooperation with the backlight 20 to achieveincreased efficiency.

Plural retarders 300 are arranged between the reflective additionalpolariser 318 and the display polariser 210. As for FIG. 1A, the pluralretarders 300 comprise: a switchable guest-host liquid crystal retarder301 comprising a layer 314 of liquid crystal material arranged betweenthe display polariser 210 and the reflective additional polariser 318;and a passive compensation retarder 330. Thus the reflective additionalpolariser 318 is arranged on the input side of the input displaypolariser 210 between the input display polariser 210 and the backlight20 and the plural retarders 300 are arranged between the reflectiveadditional polariser 318 and the input display polariser 210.

The electric vector transmission direction 319 of the reflectiveadditional polariser 318 is parallel to the electric vector transmissiondirection 211 of input polariser 210 to achieve the switchabledirectional properties as will be described hereinbelow.

In alternative embodiments the additional polariser 318 may compriseboth a reflective polariser and a linear absorbing polariser or maycomprise only a linear absorbing polariser.

The reflective additional polariser 318 may for example be a multilayerfilm such as DBEF™ from 3M Corporation, or may be a wire grid polariser.Advantageously display efficiency may be improved due to light recyclingfrom the polarised reflection from the polariser 372. Further cost andthickness may be reduced in comparison to using both a linear absorbingpolariser and a reflective polariser as additional polariser 318.

In comparison to the arrangement of FIG. 1A, FIG. 2A may provideimproved front of screen image contrast due to the reduced number oflayers between the pixels 220, 222, 224 and an observer.

It may be desirable to provide a display with a fixed privacy mode ofoperation.

FIG. 2B is a schematic diagram illustrating in side perspective view anoptical stack of a directional display device comprising a backlight 20,a rear passive guest-host liquid crystal retarder 340, and atransmissive spatial light modulator 48.

FIG. 2C is a schematic diagram illustrating in side perspective view anoptical stack of a directional display device comprising a backlight 20,a rear switchable guest-host retarder 300, and a transmissive spatiallight modulator 48 wherein the additional polariser 318 comprises alinear absorbing polariser. In comparison to the reflective additionalpolariser 318 of FIG. 2A, the dichroic additional polariser 318 does notrecycle high angle light into the backlight and thus may reduce theoff-axis luminance in comparison to the arrangement of FIG. 2A.Advantageously privacy performance is improved.

The operation of the guest-host liquid crystal retarder 301 will now bedescribed with reference to the guest-host liquid crystal retarder 301shown in FIG. 3A. The guest-host liquid crystal retarder 301 has thesame structure as described above but in this example the guest-hostretarder 301 is arranged on the output side of the spatial lightmodulator 48, the output display polariser 218 being between theguest-host retarder 301 and the spatial light modulator 48. In general,the guest-host retarder 301 may be on either side of the spatial lightmodulator 48 and the same operation is provided as described below.

FIG. 3A is a schematic diagram illustrating in side perspective view aswitchable guest-host liquid crystal retarder 301 in a privacy mode ofoperation with a first drive voltage V1; FIG. 3B is a schematic graphillustrating the variation of output transmission with polar directionfor the transmitted light rays in FIG. 3A; FIG. 3C is a schematic graphillustrating the simulated variation of output transmission with lateralangle at zero elevation with varying concentration of dichroic dye forthe transmitted light rays in FIG. 3A; and FIG. 3D is a schematic graphillustrating the simulated variation of normalised output transmissionwith lateral angle at zero elevation with varying concentration ofdichroic dye for the transmitted light rays in FIG. 3A, with theparameters described in TABLE 1.

TABLE 1 Passive compensation retarder(s) 330 Guest-host liquid crystalretarder Δn.d/ Order Guest-host LC tilt/ Δn.d/ Additional FIG. Mode Typenm parameter concentration deg nm polariser 3B Privacy None 0 0.8   1%90 810 No 3C&3D profile 750 3C&3D 0.75% profile 752 3C&3D  0.5% profile754 3F Wide  0.5%  0

In FIG. 3A and other schematic diagrams below, some layers of theoptical stack are omitted for clarity. For example the switchableguest-host liquid crystal retarder 301 is shown omitting the substrates312, 316.

The guest-host liquid crystal retarder 301 comprises guest material 414Bwhich is an anisotropic absorber and host material 414A which is aliquid crystal material. Guest material 414B may be a dichroic dye orpleochroic dye and typically may be a positive dichroic dye, withabsorption of polarisation state that is parallel to the long axis ofthe dichroic guest material 414B.

The dichroic dye is oriented by the aligned liquid crystal material 414Awhen voltage V1 is applied such that in at least one state of thematerial as illustrated in FIG. 3A the optical axis direction of theguest-host liquid crystal retarder 301 has an alignment componentperpendicular to the plane of the retarder 301.

In operation light ray 420 that is propagating in the x-z plane 421 hasa linear polarisation component imparted by display polariser 218. Suchpolarisation component is incident on a molecule 714 of the guestmaterial 414B at an orientation that is orthogonal to the absorptionaxis of the molecule 714. Thus light rays 420 in the elevation directionare substantially transmitted.

By way of comparison light ray 422 that is propagating in the y-z plane423 has a polarisation component that is parallel to the absorption axisof the dichroic dye molecule 714, and thus undergoes some absorption.Thus light rays 420 in the lateral direction are substantiallytransmitted.

FIG. 3C and FIG. 3D illustrate that as the guest-host concentration isincreased, the head-on luminance reduces, as a consequence of the orderparameter of the liquid crystal molecules limiting the alignment to thepolariser 218 of the absorbing dichroic dye molecules that can beachieved.

Advantageously, reduced off-axis luminance can be achieved, providingincreased privacy performance.

Desirable ranges for guest-host materials have been established by meansof simulation of retarder stacks and experiment with display opticalstacks.

In order to achieve desirable off-axis luminance reduction whileminimising absorption, the volume of the guest material may compriseless than 3%, preferably less than 2% and most preferably less than 1%of the volume of the host material. Alternatively, in order to achievedesirable off-axis luminance reduction while minimising absorption, theweight of the guest material may comprise less than 3%, preferably lessthan 2% and most preferably less than 1% of the weight of the hostmaterial. Advantageously reducing the guest concentration providesincreased efficiency in the direction normal to the retarder 301 whileproviding some off-axis luminance reduction.

The on-axis extinction coefficient of the guest-host liquid crystalretarder 301 may be determined by aligning the retarder 301 to a linearabsorbing polariser that has an electric vector transmission directionthat is (i) perpendicular and (ii) parallel with the absorption axis ofthe retarder 301. The extinction coefficient is the ratio of themeasurements for the perpendicular and parallel orientations. Theon-axis extinction coefficient of the guest-host liquid retarder 301 inat least one state of operation is at least 60%, preferably at least 80%and most preferably at least 90%.

FIG. 3E is a schematic diagram illustrating in side perspective view aswitchable guest-host liquid crystal retarder in a wide angle mode ofoperation with a second drive voltage V2; and FIG. 3F is a schematicgraph illustrating the variation of output transmission with polardirection for the transmitted light rays in FIG. 3E.

The guest material comprises a positive dichroic material or a positivepleochroic material and the optical axis of the guest-host liquidcrystal layer has an alignment component in the plane of the guest-hostliquid crystal layer that is orthogonal to the electric vectortransmission direction of the display polariser. Thus the transmissionaxis of the host material 414B is aligned to the transmission axis ofthe linear absorbing polariser 218. The field of the view of the inputlight is substantially unmodified by the dichroic guest material 414B.Advantageously a wide angle mode may be provided.

It may be desirable to provide a passive guest-host liquid crystalretarder 340.

FIG. 3G is a schematic diagram illustrating in side view a passiveguest-host liquid crystal retarder. The guest-host liquid crystalretarder is a cured liquid crystal layer.

Such a retarder may be provided in a first step by providing ahomeotropic alignment layer 342 on the surface of a substrate 344.Substrate 344 is a transparent substrate that may be birefringentsubstrate to provide a compensation retarder 330 or may be a reflectiveadditional polariser 318 similar to that illustrated in FIG. 2A forexample.

In a second step guest-host liquid crystal material 414A, 414B isprovided on the homeotropic alignment layer 342 by means of spinning,slot coating or other known coating method. The upper surface mayself-align with homeotropic alignment to air, by means of surfacetension forces for example. The liquid crystal material 414A may beprovided by a curable liquid crystal material such as a reactivemesogen.

In a third step, a UV light source 345 is provided to illuminate theguest-host liquid crystal material 414A, 414B, to provide a crosslinkages 346 between the liquid crystal molecules 414A.

Such elements 340 can also be incorporated in other embodimentsdescribed herein to provide a contribution to off-axis luminancereduction. Advantageously a low cost passive view angle control elementmay be provided with low thickness, and off-axis stray light reductionmay be increased.

FIG. 3H is a schematic diagram illustrating in side view a passiveguest-host liquid crystal retarder wherein the guest material 414Bcomprises silver nanowires. Alternatively or additionally to theabsorbing dichroic materials described, the anisotropic material 414Bmay comprise metallic nanomaterials that may be nanowires, nanorods,nanoplatelets or other nanoscale anisotropic particles.

In comparison to the absorbing dichroic materials described elsewhere,the nanoparticles may provide some reflective properties that ispolarisation dependent, in a similar manner to wire grid polarisers,although with homeotropic alignment introduced by the liquid crystalalignment. In particular the complex refractive index of the layer mayprovide the effect of a bulk specular reflector for a first polarisationcomponent 480 and transmit the orthogonal polarisation component 482.On-axis incident light rays 400 may be transmitted by the guest-hostliquid crystal material 414A, 414B for both polarisation components 480,482 but off-axis light rays 401 may be reflected for polarisationcomponent 480.

The metallic nanowires 414B may further comprise an electricallyinsulating and optically transparent layer 474 that prevents anelectrical path between the electrodes 413, 415. This may be achieved bychemical treatment or processing so that an optically transparentelectrically insulating coating or layer 474 is present on all or justthe end part of the nanowires. The chemical treatment or processing mayfor example comprise oxidation of nanowires, which may be aluminium.This achieves off-axis reflection from metal nanowires with essentiallyno DC electrical conductivity path within the liquid crystal material414.

FIG. 3I is a schematic diagram illustrating in perspective side view adisplay comprising a passive guest-host liquid crystal retarder 340wherein the guest material 414B comprises silver nanowires; and anadditional polariser 318 with the guest-host liquid crystal retarder 340between the output display polariser 218 and the additional polariser318. The substrate 344 of the passive guest-host liquid crystal retarder340 may further comprise passive retarder 330 in order to reduceoff-axis luminance over an increased field of view as describedelsewhere herein.

In operation, light rays 400 from the spatial light modulator 48 aretransmitted whereas off-axis light rays have a modified polarisationstate and are absorbed by the additional polariser 318 for off-axislocations. Further the silver nanowires provide reflection of light rays401 from ambient light source 604.

Ambient reflections increase the perceived background level of the imageas seen by an observer and thus reduce perceived image contrast.Advantageously privacy performance is increased.

Returning to the description of FIG. 2B, the passive guest-host retarder340 may alternatively be provided as the type of FIG. 3H. Light rays 401that are reflected into the backlight may be recirculated rather thanabsorbed. Advantageously, increased efficiency may be obtained inaddition to some collimation of the light rays from the backlight 20.

It would be desirable to increase the off-axis luminance reduction whileachieving high on-axis transmission.

FIG. 4A is a schematic diagram illustrating in side perspective view aswitchable guest-host liquid crystal retarder 301 and an additionalpolariser 318 in a privacy mode of operation. In comparison to FIG. 3A,additional polariser 318 provides increased off-axis absorption andmodifies the nature of the output transmission profile.

FIG. 4B is a schematic graph illustrating the simulated variation ofoutput transmission with polar direction for the transmitted light raysin FIG. 4A with no guest dichroic dye material. For convenience, theabsorption of the additional polariser 318 has not been included in thepresent FIGURES.

FIG. 4C is a schematic graph illustrating the simulated variation ofoutput transmission with polar direction for the transmitted light raysin FIG. 4A with a first concentration of guest dichroic dye material;FIG. 4D is a schematic graph illustrating the simulated variation ofoutput transmission with polar direction for the transmitted light raysin FIG. 4A with a second concentration of guest dichroic dye material;FIG. 4E is a schematic graph illustrating the simulated variation ofoutput transmission with lateral angle at zero elevation with varyingconcentration of dichroic dye for the transmitted light rays in FIG. 4A;and FIG. 4F is a schematic graph illustrating the simulated variation ofnormalised output transmission with lateral angle at zero elevation withvarying concentration of dichroic dye for the transmitted light rays inFIG. 4A, with the parameters described in TABLE 2.

TABLE 2 Passive compensation retarder(s) Guest-host liquid crystalretarder Δn.d/ Order Guest-host LC tilt/ Δn.d/ Additional FIG. Mode Typenm parameter concentration deg nm polariser 4B Privacy None 0 0.8   0%65 810 Yes 4E & 4F profile 758 4C 0.5% 4E & 4F profile 757 4D 1.0% 4E &4F profile 756

In comparison to the output of FIG. 4B, the dichroic dye 414Badvantageously reduces the visibility of the ‘bulls-eye’ structure andincreases the angular field of view over which privacy performance ismaintained. Further in comparison to the arrangement of FIG. 3A,off-axis transmission is further reduced.

It would be desirable to further reduce the off-axis visibility of thedisplay in privacy mode of operation while maintaining wide angleoperation. The operation of the display of FIG. 1A in wide angle moderepresenting a first state will now be further described, in which anadditional compensation retarder 330 is provided.

FIG. 4G is a schematic diagram illustrating in perspective side view anarrangement of the switchable guest-host retarder 300 in a wide anglemode of operation.

The switchable guest-host liquid crystal retarder 301 comprises twosurface alignment layers disposed adjacent to the liquid crystalmaterial 414 on opposite sides thereof and arranged to providehomeotropic alignment at the adjacent liquid crystal material 414. Asdescribed above, the liquid crystal material 414 may be provided with apretilt, for example 88 degrees from the horizontal to remove degeneracyof liquid crystal material 414 alignment.

The passive compensation retarder 330 comprises a negative C-plateretarder having an optical axis that is a fast axis perpendicular to theplane of the retarder. Thus the material 430 of the C-plate retarder mayhave a negative dielectric anisotropy. C-plates may comprise transparentbirefringent materials such as: polycarbonates or reactive mesogens thatare cast onto a substrate that provides homeotropic alignment forexample; Zeonex™ Cyclo Olefin Polymer (COP); discotic polymers; andNitto Denko double stretched polycarbonates.

Such arrangements not incorporating dye materials 414B are illustratedin U.S. Patent Publ. No. 2019-0086706, which is herein incorporated byreference in its entirety.

The present embodiments achieve substantially similar off-axis luminancereductions due to the bulk retardance properties of the host liquidcrystal material 414A. Further the off-axis luminance reductions due tothe aligned guest dye materials 414B are provided in such embodiments.Advantageously off-axis luminance may be further reduced.

FIG. 4H is a schematic diagram illustrating a graph of liquid crystaldirector angle 407 against fractional location 440 through theswitchable liquid crystal retarder cell, where the fractional location440 varies between 0 for a location at the surface alignment layer 409and 1 for a location at the surface alignment layer 411.

For a vertically aligned state with no voltage applied as illustrated inFIG. 4G, the liquid crystal directors are at a tilt 407 of 88 degreesthrough the thickness of the cell as indicated by tilt profile 442. Thetilt profile for the layer 314 may be the same as the profile 442. Thecompensation retarder 330 may provide correction for the pretiltdirection of the switchable guest-host liquid crystal retarder 301. Thecompensation retarder 330 may alternatively have a uniform tilt angle of90 degrees, such difference from the pretilt of the liquid crystal layerproviding only small difference in off-axis viewing properties.

Thus the off-axis retardance of the compensation retarder 330 issubstantially equal and opposite to the off-axis retardance of theswitchable guest-host liquid crystal retarder 301 when no voltage isapplied.

FIG. 4I is a schematic diagram illustrating in side view propagation ofoutput light from the spatial light modulator 48 through the opticalstack of FIG. 1A in a wide angle mode of operation; and FIG. 4J is aschematic graph illustrating the variation of output transmission withpolar direction for the transmitted light rays in FIG. 4I in a wideangle mode of operation.

An ideal compensated switchable retarder 300 comprises compensationretarder 330 in combination with a variable switchable guest-host liquidcrystal retarder 301 wherein the dielectric constants, anisotropy anddispersion of anisotropy of the compensation retarder 330 have the equaland opposite dielectric constants, anisotropy and dispersion ofanisotropy to that of the layer 314. The retardance of the passivecompensation retarder 330 is equal and opposite to the retardance of theswitchable guest-host liquid crystal retarder 301.

Such an ideal compensated switchable retarder achieves compensation fortransmitted light in a first wide angle state of the layer 314 of liquidcrystal material 414 for all polar angles; and narrow field of view in alateral direction in a second privacy state of the switchable guest-hostliquid crystal retarder 301.

Further the optical axis of compensation retarder 330 has the samedirection as that of the optical axis of the guest-host liquid crystalretarder 301 in its wide angle state. Such a compensation retarder 330cancels out the retardation of the liquid crystal retarder for allviewing angles, and provides an ideal wide angle viewing state with noloss of luminance for all viewing directions.

The wide angle transmission polar profile for non-ideal materialselections will now be described.

The illustrative embodiments of the present disclosure illustratecompensation retarders 330 that may not exactly compensate theretardation of the switchable guest-host liquid crystal retarder 301because of small differences in material properties that are typical forthe retarders 330, 301. However, advantageously such deviations aresmall and high performance wide and narrow angle states can be achievedwith such deviations that may be close to ideal performance.

Thus when the switchable guest-host liquid crystal retarder 301 is in afirst state of said two states, the switchable guest-host retarder 300provides no overall transformation of polarisation component 360, 361 tooutput light rays 400 passing there through perpendicular to the planeof the switchable retarder or at an acute angle to the perpendicular tothe plane of the switchable retarder, such as for light rays 402.

Polarisation component 362 is substantially the same as polarisationcomponent 360 and polarisation component 364 is substantially the sameas polarisation component 361. Thus the angular transmission profile ofFIG. 4J is substantially uniformly transmitting across a wide polarregion.

In other words, when the layer of liquid crystal material 414 is in thefirst orientation state of said two orientation states, the pluralretarders 330, 301 provide no overall retardance to light passingtherethrough perpendicular to the plane of the retarders or at an acuteangle to the perpendicular to the plane of the retarders 330, 301.

Advantageously the variation of display luminance with viewing angle inthe first state is substantially unmodified. Multiple users mayconveniently view the display from a wide range of viewing angles.

Further the wide angle profile of FIG. 3F is provided with such anarrangement due to the action of the guest dichroic material 414B. Thusthe wide angle mode performance may be substantially unmodified by thedichroic material 414B. Advantageously a display that can be observedfrom a large field of view may be provided.

The operation of the guest-host retarder 300 and additional polariser318 in a narrow angle mode for example for use in a privacy mode ofoperation will now be described.

FIG. 5A is a schematic diagram illustrating in perspective side view anarrangement of retarders 300 in a privacy mode of operation comprising anegative C-plate passive compensation retarder 330 and homeotropicallyaligned switchable guest-host liquid crystal retarder 301 in a privacymode of operation.

The guest-host liquid crystal retarder 301 further comprises transparentelectrodes 413, 415 such as ITO electrodes arranged on opposite sides ofthe switchable guest-host liquid crystal retarder 301. Electrodes 413,415 control the switchable guest-host liquid crystal retarder 301 byadjusting the voltage being applied by the electrodes 413, 415 to theguest-host liquid crystal retarder 301. The applied voltage is capableof switching host material between at least two states, in one of whichstates the optical axis of the guest-host liquid crystal retarder has analignment component perpendicular to the plane of the guest-host liquidcrystal retarder 301.

Control system 352 is arranged to control the voltage applied by voltagedriver 350 across the electrodes 413, 415 of the switchable guest-hostliquid crystal retarder 301.

Returning to FIG. 4H, when a voltage is applied the splayed tilt profile444 of is provided for switchable guest-host liquid crystal retarder 301such that the retardance of the layer 314 of liquid crystal material 414is modified.

The direction of optimum privacy performance may be adjusted in responseto observer position by control of the drive voltage. In another use orto provide controlled luminance to off-axis observers for example in anautomotive environment when a passenger or driver may wish somevisibility of the displayed image, without full obscuration, by means ofintermediate voltage levels.

FIG. 5B is a schematic diagram illustrating in side view propagation ofoutput light from the spatial light modulator 48 through the opticalstack of FIG. 1A in a privacy mode of operation wherein the switchableguest-host liquid crystal retarder 301 is oriented by means of anapplied voltage.

In the present embodiments, the compensated switchable liquid crystalretarder 330 may be configured, in combination with the displaypolariser 210, 218, 316 and the additional polariser 318, to have theeffect that the luminance of light output from the display device at anacute angle to the optical axis (off-axis) is reduced, i.e. compared tothe retarder not being present. The compensated switchable liquidcrystal retarder 330 may also be configured, in combination with thedisplay polariser 210, 218, 316 and the additional polariser 318, tohave the effect that the luminance of light output from the displaydevice along the optical axis (on-axis) is not reduced, i.e. compared tothe retarder not being present.

Polarisation component 360 from the output display polariser 218 istransmitted by output display polariser 218 and incident on switchableguest-host retarder 300. On-axis light has a polarisation component 362that is unmodified from component 360 while off-axis light has apolarisation component 364 that is transformed by retarders ofswitchable guest-host retarder 300. At a minimum, the polarisationcomponent 361 is transformed to a linear polarisation component 364 andabsorbed by additional polariser 318. More generally, the polarisationcomponent 361 is transformed to an elliptical polarisation component,that is partially absorbed by additional polariser 318.

Thus when the retarder switchable guest-host liquid crystal retarder 301is in the second orientation state of said two orientation states, theplural retarders 301, 330 provide no overall retardance to light passingtherethrough along an axis perpendicular to the plane of the retarders,but provides a non-zero overall retardance to light passing therethroughfor some polar angles 363 that are at an acute angle to theperpendicular to the plane of the retarders 301, 330.

In other words when the switchable guest-host liquid crystal retarder301 is in a second state of said two states, the switchable compensatedretarder 330 provides no overall transformation of polarisationcomponent 360 to output light rays 400 passing therethrough along anaxis perpendicular to the plane of the switchable retarder 301, butprovides an overall transformation of polarisation component 361 tolight rays 402 passing therethrough for some polar angles which are atan acute angle to the perpendicular to the plane of the retarders 301,330.

An illustrative material system will be described for narrow angleoperation.

FIG. 5C, FIG. 5D and FIG. 5E are schematic graphs illustrating thevariation of output transmission with polar direction for thetransmitted light rays in FIG. 5B, with the parameters described inTABLE 3.

TABLE 3 Passive compensation retarder(s) Guest-host liquid crystalretarder Δn.d/ Order Guest-host LC tilt/ Δn.d/ Additional FIG. Mode Typenm parameter concentration deg nm polariser 5C Privacy Negative C −7000.8   0% 65 810 Yes 5D 0.5% 5E 1.0% 4J Wide 0.0% 0

In the present embodiments, desirable ranges for retardations andvoltages have been established by means of simulation of retarder stacksand experiment with display optical stacks.

The switchable liquid crystal retarder 300 comprises a first surfacealignment layer 409 disposed on a first side of the layer of liquidcrystal material 414, and a second surface alignment layer 411 disposedon the second side of the layer of liquid crystal material 414 oppositethe first side; wherein the first surface alignment layer 409 is ahomeotropic alignment layer and the second surface alignment layer 411is a homeotropic alignment layer, wherein the layer of liquid crystalmaterial has an retardance for light of a wavelength of 550 nm between500 nm and 1000 nm, preferably between 600 nm and 900 nm and mostpreferably between 700 nm and 850 nm.

When the passive compensation retarder 330 comprises a retarder havingan optical axis perpendicular to the plane of the retarder, the passiveretarder has a retardance for light of a wavelength of 550 nm between−300 nm and −900 nm, preferably between −450 nm and −800 nm and mostpreferably between −500 nm and −725 nm.

In comparison to the output of FIG. 4B the polar distribution of lighttransmission illustrated in FIG. 5C modifies the polar distribution ofluminance output from the underlying spatial light modulator 48 andwhere applicable the backlight 20.

Considering now the addition of guest dichroic dye material 414B, incomparison to the output of FIG. 4C the profile of FIG. 5D providesincreased luminance reduction over a wider field of view.

Advantageously off-axis luminance may be further reduced. A privacydisplay is provided that has low luminance to an off-axis snooper whilemaintaining high luminance for an on-axis observer. A large polar regionis provided over which the luminance of the display to an off-axissnooper is reduced. Further the on-axis luminance is substantiallyunaffected for the primary display user in privacy mode of operation.

The voltage applied across the electrodes is zero for the firstorientation state and non-zero for the second orientation state.Advantageously the wide mode of operation may have no additional powerconsumption, and the failure mode for driving of the switchableguest-host liquid crystal retarder 301 is for wide angle mode.

In the further exemplary embodiments described below comprisingguest-host liquid crystal retarders, such guest material may serve tofurther reduce off-axis luminance in privacy mode while having a smallereffect on the wide angle mode of operation. Unless otherwise stated, thefield-of-view profiles below are provided for no added guest material414B, however the trends as illustrated above in regards to the additionof additional polarisers 318, compensation retarders 330 and varyingconcentration of guest material 414B apply to provide reduced off-axisluminance.

The operation of the privacy mode of the display of FIG. 1A will now bedescribed further.

FIG. 6A is a schematic diagram illustrating in front perspective viewobservation of transmitted output light for a display operating inprivacy mode. Display device 100 may be provided with white regions 603and black regions 601. A snooper may observe an image on the display ifluminance difference between the observed regions 601, 603 can beperceived. In operation, primary user 45 observes a full luminanceimages by rays 400 to viewing locations 26 that may be optical windowsof a directional display. Snooper 47 observes reduced luminance rays 402in viewing locations 27 that may be optical windows of a directionaldisplay. Regions 26, 27 further represent on-axis and off-axis regionsof FIG. 5C.

FIG. 6B is a schematic diagram illustrating in front perspective viewsthe appearance of the display of FIG. 1A operating in privacy mode 1with luminance variations as illustrated in FIG. 5C. Thus upper viewingquadrants 530, 532, lower viewing quadrants 534, 536 and lateral viewingpositions 526, 528 provide reduced luminance, whereas up/down centralviewing regions 522, 520 and head-on viewing provides much higherluminance.

It may be desirable to provide controllable display illumination in anautomotive vehicle.

FIG. 6C is a schematic diagram illustrating in side view an automotivevehicle with a switchable directional display 100 arranged within thevehicle cabin 602 of an automotive vehicle 600 for both entertainmentand sharing modes of operation. Light cone 610 (for example representingthe cone of light within which the luminance is greater than 50% of thepeak luminance) may be provided by the luminance distribution of thedisplay 100 in the elevation direction and is not switchable.

FIG. 6D is a schematic diagram illustrating in top view an automotivevehicle with a switchable directional display 100 arranged within thevehicle cabin 602 in an entertainment mode of operation and operates ina similar manner to a privacy display. Light cone 612 is provided with anarrow angular range such that passenger 606 may see the display 100whereas driver 604 may not see an image on the display 100.Advantageously entertainment images may be displayed to the passenger606 without distraction to the driver 604.

FIG. 6E is a schematic diagram illustrating in top view an automotivevehicle with a switchable directional display 100 arranged within thevehicle cabin 602 in a sharing mode of operation. Light cone 614 isprovided with a wide angular range such that all occupants may perceivean image on the display 100, for example when the display is not inmotion or when non-distracting images are provided.

FIG. 6F is a schematic diagram illustrating in top view an automotivevehicle with a switchable directional display 100 arranged within thevehicle cabin 602 for both night-time and day-time modes of operation.In comparison to the arrangements of FIGS. 6C-E, the optical output isrotated so that the display elevation direction is along an axis betweenthe driver 604 and passenger 606 locations. Light cone 620 illuminatesboth driver 604 and passenger 606.

FIG. 6G is a schematic diagram illustrating in side view an automotivevehicle with a switchable directional display 100 arranged within thevehicle cabin 602 in a night-time mode of operation. Thus the displaymay provide a narrow angular output light cone 622. Stray light thatilluminates internal surfaces and occupants of the vehicle cabin 602 andcause distraction to driver 604 may advantageously be substantiallyreduced. Both driver 604 and passenger 606 may advantageously be able toobserve the displayed images.

FIG. 6H is a schematic diagram illustrating in side view an automotivevehicle with a switchable directional display 100 arranged within thevehicle cabin 602 in a day-time mode of operation. Thus the display mayprovide a narrow angular output light cone 624. Advantageously thedisplay may be conveniently observed by all cabin 602 occupants.

The displays 100 of FIGS. 6C-H may be arranged at other vehicle cabinlocations such as driver instrument displays, centre console displaysand seat-back displays.

FIGS. 7A-D are schematic diagrams illustrating the variation of outputtransmission with polar direction for four different drive voltages from2.05V to 2.35V in 0.1V increments. Thus the applied voltage may providecontrol of the luminance field-of-view minima locations in the privacymode of operation. Further the luminance minima may be controlledbetween an elevation that is zero or less to elevations that are in theupper quadrants of the polar profile.

Returning to the discussion of the present embodiments, furtherarrangements of compensated switchable retarders 300 will now bedescribed.

FIG. 8A is a schematic diagram illustrating in perspective side view anarrangement of a switchable retarder in a privacy mode of operationcomprising crossed A-plate passive compensation retarders 308A, 308B andhomeotropically aligned switchable guest-host liquid crystal retarder301; and FIG. 8B is a schematic diagram illustrating in perspective sideview an arrangement of a switchable compensated retarder in a privacymode of operation comprising crossed A-plate passive compensationretarders and homeotropically aligned switchable liquid crystalretarder.

In comparison to the arrangement of FIG. 4G and FIG. 5A, thecompensation retarder 330 may alternatively comprise a pair of retarders308A, 308B which have optical axes in the plane of the retarders thatare crossed. The compensation retarder 330 thus comprises a pair ofretarders 308A, 308B that each comprise a single A-plate.

The pair of retarders 308A, 308B each comprise plural A-plates havingrespective optical axes 309A, 309B aligned at different angles withrespect to each other. The pair of retarders have optical axes 309A,309B that each extend at 45° with respect to an electric vectortransmission direction that is parallel to the electric vectortransmission direction 211 of the input display polariser 210 in thecase that the additional polariser 318 is arranged on the input side ofthe input display polariser or is parallel to the electric vectortransmission direction 219 of the output display polariser 218 in thecase that the additional polariser 318 is arranged on the output side ofthe input display polariser 218.

FIG. 9A is a schematic graph illustrating the variation of outputtransmission with polar direction for transmitted light rays in FIG. 8Ain a wide angle mode of operation; and FIG. 9B is a schematic graphillustrating the variation of output transmission with polar directionfor transmitted light rays in FIG. 8B in a privacy mode of operationprovided by the illustrative embodiment of TABLE 4.

TABLE 4 Passive compensation retarder(s) Active LC retarder Δn.d/Alignment Pretilt/ Δn.d/ Voltage/ FIG. Mode Type nm layers deg nm Δε V9A Wide Crossed A +650 @ 45°  Homeotropic 88 810 −4.3 0 9D Privacy +650@ 135° Homeotropic 88 2.3

When the passive compensation retarder 330 comprises a pair of retarderswhich have optical axes in the plane of the retarders that are crossed,each retarder of the pair of retarders has a retardance for light of awavelength of 550 nm between 300 nm and 800 nm, preferably between 500nm and 700 nm and most preferably between 550 nm and 675 nm.

Advantageously A-plates may be more conveniently manufactured at lowercost than for the C-plate retarder of FIG. 4G and FIG. 5A. Further azero voltage state may be provided for the wide angle mode of operation,minimising power consumption during wide angle operation.

In the present embodiments, ‘crossed’ refers to an angle ofsubstantially 90° between the optical axes of the two retarders in theplane of the retarders. To reduce cost of retarder materials, it isdesirable to provide materials with some variation of retarderorientation due to stretching errors during film manufacture forexample. Variations in retarder orientation away from preferabledirections can reduce the head-on luminance and increase the minimumtransmission. Preferably the angle 310A is at least 35° and at most 55°,more preferably at least 40° and at most 50° and most preferably atleast 42.5° and at most 47.5°. Preferably the angle 310B is at least125° and at most 145°, more preferably at least 130° and at most 135°and most preferably at least 132.5° and at most 137.5°.

During mechanical distortion, such as when touching the display, thehomeotropically aligned liquid crystal retarders 301 of FIGS. 9A-9B mayhave undesirably long recovery times creating visible misalignmentartefacts. It would be desirable to provide fast recovery times aftermechanical distortion.

FIGS. 10A-10B are schematic diagrams illustrating in perspective sideview an arrangement of a switchable retarder in a wide angle and privacymode of operation respectively comprising a homogeneously alignedswitchable liquid crystal retarder comprising liquid crystal material414 with a positive dielectric anisotropy and a passive negative C-plateretarder 330 for first and second drive voltages respectively.

The switchable liquid crystal retarder further comprises surfacealignment layers 431, 433 disposed adjacent to the layer of liquidcrystal material 414 and each arranged to provide homogeneous alignmentin the adjacent liquid crystal material. In other words, the switchableliquid crystal retarder comprises two surface alignment layers 431, 433disposed adjacent to the layer of liquid crystal material 414 and onopposite sides thereof and each arranged to provide homogeneousalignment in the adjacent liquid crystal material 414.

FIG. 10C is a schematic diagram illustrating a graph of liquid crystaldirector angle 407 against fractional location 440 through theswitchable guest-host liquid crystal retarder 301 of FIG. 10A forvarious different applied voltages. FIG. 10C differs from FIG. 4Hwherein the pretilt angle is small and increases with applied voltage.Profile 441 illustrates liquid crystal material 414 tilt angle for 0Vapplied voltage, tilt profile 443 illustrates director orientations for2.5V and tilt profile 445 illustrates director orientations for 5V. Thusthe liquid crystal layers are typically splayed in desirable switchedstates, and compensated by the compensation retarders 330. Increasingthe voltage above 2.5V to 10V progressively reduces the thickness of theretarder 301 in which splay is present, and advantageously increases thepolar field of view over which the transmission is maximised.

Resolved component 419 a, 419 b of liquid crystal tilt compared to thedirection perpendicular to the plane of the retarder is substantiallyhigher than components 417 a, 417 b of FIG. 5A.

The increased magnitude of resolved component 419 a, 419 b may provideincreased restoring force after mechanical distortion in comparison tothe arrangement of FIG. 8A for example. Sensitivity to mechanicaldistortions such as during touching the display may advantageously bereduced.

The voltage of operation may be reduced below 10V for acceptable wideangle field of view, reducing power consumption; and reducing cost andcomplexity of electrical driving.

FIGS. 11A-11C are schematic graphs illustrating the variation of outputtransmission with polar direction for transmitted light rays ofswitchable compensated retarder comprising a homogeneously alignedguest-host liquid crystal retarder 301 and a passive negative C-platecompensation retarder 330, similar to the display device of FIGS. 10Aand 10B, in a privacy mode and two different wide angle modes fordifferent drive voltages comprising the embodiments illustrated in TABLE5.

TABLE 5 Passive compensation retarder(s) Active LC retarder Δn.d/Alignment Pretilt/ Δn.d/ Voltage/ FIG. Mode Type nm layers deg nm Δε V11A Privacy Negative C −500 Homogeneous 2 750 +13.2 2.3 11B WideHomogeneous 2 5.0 11C Wide 10.0

Desirable ranges for optical retardance for active LC retarder 301comprising homogeneous alignment layers 431, 433 on both substrates anda passive negative C-plate compensation retarder 330 are furtherdescribed in TABLE 6.

TABLE 6 Active Minimum Typical Maximum LC layer negative C- negative C-negative C- retardance/ plate plate plate nm retardance/nm retardance/nmretardance/nm 600 −300 −400 −500 750 −350 −450 −600 900 −400 −500 −700

The switchable liquid crystal retarder 300 thus comprises a firstsurface alignment layer 431 disposed on a first side of the layer ofliquid crystal material 414, and a second surface alignment layer 433disposed on the second side of the layer of liquid crystal material 414opposite the first side; wherein the first surface alignment layer 409is a homogeneous alignment layer and the second surface alignment layeris a homogeneous alignment layer; wherein the layer of liquid crystalmaterial has a retardance for light of a wavelength of 550 nm in a rangefrom 500 nm to 1000 nm, preferably in a range from 600 nm to 850 nm andmost preferably in a range from 700 nm to 800 nm. Thus when the firstand second alignment layers are each homogeneous alignment layers andwhen the passive compensation retarder 330 comprises a retarder havingan optical axis perpendicular to the plane of the retarder, the passiveretarder has a retardance for light of a wavelength of 550 nm in a rangefrom −300 nm to −700 nm, preferably in a range from −350 nm to −600 nmand most preferably in a range from −400 nm to −500 nm.

Advantageously off-axis privacy can be provided by means of luminancereduction and privacy level increase over wide polar regions. Furtherresistance to visual artefacts arising from flow of liquid crystalmaterial in the layer 314 may be improved in comparison to homeotropicalignment.

Various other configurations of the optical structure and driving ofFIG. 10A will now be described.

Operation at 5V provides lower power consumption and lower costelectronics while achieving acceptable luminance roll-off in wide anglemode. Field of view in wide angle mode can further be extended byoperation 10V.

FIG. 12 is a schematic diagram illustrating in perspective side view anarrangement of a switchable compensated retarder in a privacy mode ofoperation, the arrangement comprising crossed A-plate passivecompensation retarders 308A, 308B and homogeneously aligned switchableguest-host liquid crystal retarder 301; and FIGS. 13-15 are schematicgraphs illustrating the variation of output transmission with polardirection for transmitted light rays of switchable compensated retarder301 comprising a homogeneously aligned liquid crystal material 414 andpassive crossed A-plate retarders 308A, 308B, in a privacy mode and awide angle mode for different drive voltages comprising the respectiveembodiments illustrated in TABLE 7.

TABLE 7 Passive compensation retarder(s) Active LC retarder Δn.d/Alignment Pretilt/ Δn.d/ Voltage/ FIG. Mode Type nm layers deg nm Δε V13 Privacy Crossed A +500 @ 45°  Homogeneous 2 750 +13.2 2.3 14 Wide+500 @ 135° Homogeneous 2 5 15 Wide 10

Desirable ranges for optical retardance for active LC retarder 301comprising homogeneous alignment layers 409, 411 on both substrates andcrossed positive A-plate retarders 308A, 308B are further described inTABLE 8.

TABLE 8 Active Minimum Typical Maximum LC layer positive positivepositive retardance/ A-plate A-plate A-plate nm retardance/nmretardance/nm retardance/nm 600 +300 +400 +600 750 +350 +500 +700 900+400 +600 +800

Thus when: the first and second alignment layers are each homogeneousalignment layers; the layer of liquid crystal material has a retardancefor light of a wavelength of 550 nm in a range from 500 nm to 1000 nm,preferably in a range from 600 nm to 850 nm and most preferably in arange from 700 nm to 800 nm; and the passive compensation retarder 330comprises a pair of retarders which have optical axes in the plane ofthe retarders that are crossed, then each retarder of the pair ofretarders has an retardance for light of a wavelength of 550 nm between300 nm and 800 nm, preferably between 350 nm and 650 nm and mostpreferably between 450 nm and 550 nm.

Further crossed A-plates may be conveniently provided from low costmaterials.

By way of illustration various other example embodiments of the opticalstructure and driving of FIG. 12 will now be described. FIG. 14 and FIG.15 further illustrate that by adjustment of addressing voltage andretardances, advantageously different wide angle fields of view may beachieved.

Arrangements of optical stack structures will now be further described.

FIG. 16A and FIG. 16B are schematic diagrams illustrating in side viewspart of a display comprising a switchable compensated retarder andoptical bonding layers 380. Optical bonding layers 380 may be providedto laminate films and substrates, achieving increased efficiency andreduced luminance at high viewing angles in privacy mode. Further an airgap 384 may be provided between the spatial light modulator 48 and theswitchable guest-host retarder 301 which in this instance is arranged onthe input side of the spatial light modulator 48. To reduce wetting ofthe two surfaces at the air gap 384, an anti-wetting surface 382 may beprovided to at least one of the switchable guest-host retarder 300 orspatial light modulator 48.

The passive compensation retarder 330 may be provided between theswitchable liquid crystal layer 301 and spatial light modulator 48 asillustrated in FIG. 16A, or may be provided between the additionalpolariser 318 and switchable guest-host liquid crystal retarder 301 asillustrated in FIG. 16B. Substantially the same optical performance isprovided in both systems.

FIG. 16A illustrates that optical layers are bonded to outer sides ofthe substrates 312, 316. Advantageously, bending of the substrates 312,316 from the attached layers due to stored stresses during laminationmay be reduced and display flatness maintained.

Similarly, switchable guest-host retarder 300 may be arranged whereinthe output polariser 218 is the display polariser. Scatter that may beprovided by spatial light modulator 48, such as from phase structures atthe pixels 220, 222, 224 do not degrade the output luminance profile incomparison to arrangements wherein the switchable compensated retarder301 is arranged behind the spatial light modulator 48.

It may be desirable to provide the additional polariser with a differentelectric vector transmission direction to the electric vectortransmission direction of the display polariser.

FIG. 17 is a schematic diagram illustrating in perspective side view anarrangement of a switchable compensated retarder in a privacy mode ofoperation comprising the crossed A-plate passive compensation retarders308A, 308B and homogeneously aligned switchable guest-host liquidcrystal retarder 301, as described above but further comprising apassive rotation retarder 460.

The display polariser 218 may be provided with an electric vectortransmission direction 219, that may be for example at an angle 317 of45 degrees in the case of a twisted nematic LCD display. The additionalpolariser 318 may be arranged to provide vertically polarised light to auser whole may be wearing polarising sunglasses that typically transmitvertically polarised light.

The passive rotation retarder 460 is different to the compensationretarder 330 of the present embodiments and its operation will now bedescribed.

Passive rotation retarder 460 may comprise a birefringent material 462and be a half waveplate, with retardance at a wavelength of 550 nm of275 nm for example.

Passive rotation retarder 460 has a fast axis orientation 464 that isinclined at an angle 466 that may be 22.5 degrees to the electric vectortransmission direction 319 of the additional polariser 318. The passiverotation retarder 460 thus rotates the polarisation from the outputpolariser 218 such that the polarisation direction of the light that isincident onto the compensation retarder 308B is parallel to thedirection 319.

The passive rotation retarder 460 modifies the on-axis polarisationstate, by providing an angular rotation of the polarisation componentfrom the display polariser 218. In comparison to the compensationretarders 308A, 308B together do not modify the on-axis polarisationstate.

Further, the passive rotation retarder 460 provides a rotation ofpolarisation that may be substantially independent of viewing angle. Incomparison the compensation retarders 308A, 308B provide substantialmodifications of output luminance with viewing angle.

Advantageously a display may be provided with an output polarisationdirection 319 that is different from the display polariser polarisationdirection 219, for example to provide viewing with polarisingsunglasses.

In an alternative embodiment the separate retarder 460 may be omittedand the retardance of the retarder 308B of FIG. 11A increased to providean additional half wave rotation in comparison to the retardance ofretarder 308A. To continue the illustrative embodiment, the retardanceof retarder 308B at a wavelength of 550 nm may be 275 nm greater thanthe retardance of retarder 308A. Advantageously the number of layers,complexity and cost may be reduced.

Hybrid aligned structures comprising both homogeneous and homeotropicalignment layers will now be described.

FIG. 18 is a schematic diagram illustrating in perspective side view anarrangement of a switchable retarder in a privacy mode of operationcomprising a homogeneously and homeotropically aligned switchableguest-host liquid crystal retarder 301 comprising liquid crystalmaterial 423 and a passive negative C-plate retarder 330.

FIGS. 19-20 are schematic graphs illustrating the variation of outputtransmission with polar direction for transmitted light rays in FIG. 18in a wide angle and privacy mode of operation respectively, and providedby the arrangement of TABLE 9.

TABLE 9 Passive compensation retarder(s) Active LC retarder Δn.d/Alignment Pretilt/ Δn.d/ Voltage/ FIGURE Mode Type nm layers deg nm Δε V20 Wide Negative C −1100 Homogeneous 2 1300 +4.3 15.0 19 PrivacyHomeotropic 88 2.8 Not shown Wide Crossed A +500 @ 45°  Homeotropic 21300 +4.3 15.0 Not shown Privacy +500 @ 135° Homogeneous 88 2.8

The hybrid aligned switchable guest-host liquid crystal retarder 301 hasvariable tilt such that for a given material and cell thickness choice,reduced effective birefringence is provided. Thus the retarder designmust be adjusted to compensate in comparison to the arrangements whereinthe alignment layers are the same. The switchable liquid crystalretarder 330 comprises a first surface alignment layer 441 disposed on afirst side of the layer of liquid crystal material 423, and a secondsurface alignment layer 443 disposed on the second side of the layer ofliquid crystal material 423 opposite the first side. The first surfacealignment layer 441 is a homeotropic alignment layer arranged to providehomeotropic alignment in the adjacent liquid crystal material 423 andthe second surface alignment layer 443 is a homogeneous alignment layerarranged to provide homogeneous alignment in the adjacent liquid crystalmaterial 423.

Further, the optimum designs of retarders are related to the relativelocation of the passive compensation retarder 330 with respect to thehomeotropic and homogeneous alignment layers.

When the surface alignment layer 443 arranged to provide homogeneousalignment is between the layer of liquid crystal material 423 and thecompensation retarder 330, the layer of liquid crystal material 423 hasa retardance for light of a wavelength of 550 nm in a range from 500 nmto 1800 nm, preferably in a range from 700 nm to 1500 nm and mostpreferably in a range from 900 nm to 1350 nm. When the surface alignmentlayer 443 arranged to provide homogeneous alignment is between the layerof liquid crystal material 423 and the compensation retarder 330, thepassive compensation retarder may comprise a retarder 330 having itsoptical axis perpendicular to the plane of the retarder as shown in FIG.18, the passive retarder 330 having a retardance for light of awavelength of 550 nm in a range from −300 nm to −1600 nm, preferably ina range from −500 nm to −1300 nm and most preferably in a range from−700 nm to −1150 nm; or alternatively the passive compensation retardermay comprise a pair of retarders (not shown) which have optical axes inthe plane of the retarders that are crossed, each retarder of the pairof retarders having a retardance for light of a wavelength of 550 nm ina range from 400 nm to 1600 nm, preferably in a range from 600 nm to1400 nm and most preferably in a range from 800 nm to 1300 nm.

When the surface alignment layer 441 arranged to provide homeotropicalignment is between the layer of liquid crystal material 423 and thecompensation retarder 330, the layer of liquid crystal material 423 hasa retardance for light of a wavelength of 550 nm in a range from 700 nmto 2000 nm, preferably in a range from 1000 nm to 1700 nm and mostpreferably in a range from 1200 nm to 1500 nm. When the surfacealignment layer 441 arranged to provide homeotropic alignment is betweenthe layer of liquid crystal material 423 and the compensation retarder330, the passive compensation retarder may comprise a retarder 330having its optical axis perpendicular to the plane of the retarder asshown in FIG. 18, the passive retarder having a retardance for light ofa wavelength of 550 nm in a range from −400 nm to −1800 nm, preferablyin a range from −700 nm to −1500 nm and most preferably in a range from−900 nm to −1300 nm; or alternatively the passive compensation retardermay comprise a pair of retarders (not shown) which have optical axes inthe plane of the retarders that are crossed, each retarder of the pairof retarders having a retardance for light of a wavelength of 550 nm ina range from 400 nm to 1800 nm, preferably in a range from 700 nm to1500 nm and most preferably in a range from 900 nm to 1300 nm.

In comparison to the arrangement of FIG. 5A, the privacy mode ofoperation may advantageously achieve increased resilience to theappearance of material flow when the liquid crystal retarder is pressed.

It may be desirable to provide further control of off-axis privacy fieldof view.

FIG. 21 is a schematic diagram illustrating in perspective side view anarrangement of a switchable retarder in a privacy mode of operation,comprising: a switchable guest-host retarder 301 (in this case, ahomeotropically aligned switchable guest-host liquid crystal retarder301, but this is merely an example and may be replaced by any of theother arrangements of guest-host liquid crystal retarders disclosedherein) arranged on the output side of the spatial light modulator 48(not shown in FIG. 21) at the output of an additional polariser 318; anda switchable transmissive retarder 305 (in this case, a negative C-platepassive compensation retarder 330 and homeotropically aligned switchabletransparent liquid crystal retarder 303, but this is merely an exampleand may be replaced by any of the other arrangements of plural retardersdisclosed herein) arranged between the display polariser 218 andadditional polariser 318 with electric vector transmission direction319.

For the purposes of the present disclosure, liquid crystal retardersthat are not provided by guest-host liquid crystals are termedtransparent liquid crystal retarders as there is substantially no lightabsorption within the liquid crystal layer.

As an alternative, the additional polariser 318 may be arranged on theinput side of the input display polariser 210, in which case theadditional polariser 318 may be arranged on the input side of the inputdisplay polariser 210 between the input polariser 210 and the backlight20, and the switchable guest-host liquid crystal retarder 301 may bearranged between the additional polariser 318 and the backlight 20. As afurther alternative the retarders 301, 305 may be arranged on oppositesides of the spatial light modulator 48.

In each of these alternatives, each of the retarders 301, 305 (and/orpassive guest-host liquid crystal retarder 340 if incorporated) so havean effect similar to that of the corresponding structure in the devicesdescribed above.

The pretilt directions 307A, 309A of the alignment layers of theswitchable guest-host liquid crystal retarder 301 may have a componentin the plane of the liquid crystal layer that is aligned parallel orantiparallel or orthogonal to the pretilt directions of the alignmentlayers 307B, 309B of the switchable transparent liquid crystal retarder303. In a wide mode of operation, both switchable guest-host liquidcrystal retarders 301 and switchable transparent liquid crystal retarder303 are driven to provide a wide viewing angle. In a privacy mode ofoperation, switchable liquid crystal retarders 301, 303 may cooperate toadvantageously achieve increased luminance reduction and thus improvedprivacy in a single axis.

The retardation provided by the first switchable guest-host liquidcrystal retarder 301 and further liquid crystal retarders 303 may bedifferent. The switchable guest-host liquid crystal retarder 301 andfurther switchable transparent liquid crystal retarder 303 may be drivenby a common voltage.

The liquid crystal material 414A in the switchable guest-host liquidcrystal retarder 301 may be different to the liquid crystal material414C in the switchable transparent liquid crystal retarder 303.Chromatic variation of the polar luminance profiles illustratedelsewhere herein may be reduced, so that advantageously off-axis colourappearance is improved.

Alternatively, switchable liquid crystal retarders 301, 303 may haveorthogonal alignments so that reduced luminance is achieved in bothhorizontal and vertical directions, to advantageously achieve landscapeand portrait privacy operation.

Alternatively as illustrated, the layers 303, 301 may be provided withdifferent drive voltages. Advantageously increased control of roll-offof luminance profile may be achieved or switching between landscape andprivacy operation may be provided.

FIG. 22 is a schematic diagram illustrating in perspective side view anarrangement of switchable compensated retarder 305 arranged on theoutput of spatial light modulator 48 and a switchable guest-host liquidcrystal retarder 301 arranged on the input of the spatial lightmodulator 48.

An additional polariser 318 is arranged on the output side of the outputdisplay polariser 218 and transparent liquid crystal retarder 303 andcompensation retarder 330 arranged between the additional polariser 318and output polariser 218.

Further, a guest-host liquid crystal retarder 301 is provided betweenthe backlight 20 and input reflective polariser 208 and input linearabsorbing polariser 210 and arranged as described above.

Reflected off-axis light from the reflective polariser 208 may undergofurther absorption in the guest-host liquid crystal retarder 301 and notbe recycled in the backlight, achieving increased collimation andreducing off-axis luminance. Further reduction of off-axis luminance orlandscape-portrait operation may be provided by the transparent liquidcrystal retarder 303 and compensation retarder 330.

It may be desirable to provide both entertainment and night-time modesof operation in an automotive vehicle.

FIG. 23A is a schematic diagram illustrating in top view an automotivevehicle with a switchable directional display such as that illustratedin FIG. 22 arranged within the vehicle cabin 602 for day-time and/orsharing modes of operation; and FIG. 23B is a schematic diagramillustrating in side view an automotive vehicle with a switchabledirectional display arranged within the vehicle cabin 602 for day-timeand/or sharing modes of operation. Light cone 630, 632 is provided witha wide angular field of view and thus the display is advantageouslyvisible by multiple occupants.

FIG. 24A is a schematic diagram illustrating in top view an automotivevehicle with a switchable directional display such as that illustratedin FIG. 22 arranged within the vehicle cabin 602 for night-time and/orentertainment modes of operation; FIG. 24B is a schematic diagramillustrating in side view an automotive vehicle with a switchabledirectional display arranged within the vehicle cabin 602 for night-timeand/or entertainment modes of operation. Light cone 634, 636 is providedwith a narrow angular field of view and thus the display isadvantageously visible only by a single occupant. Advantageously straylight for night-time operation is reduced, increasing driver safety.Further, reflections of the display from windscreen 601 are reduced,minimising distraction to the driver 604.

It would be desirable to provide further reduction of off-axis luminanceby means of directional illumination from the spatial light modulator48. Directional illumination of the spatial light modulator 48 bydirectional backlights 20 will now be described.

FIG. 25A is a schematic diagram illustrating in front perspective view adirectional backlight 20, and FIG. 25B is a schematic diagramillustrating in front perspective view a non-directional backlight 20,either of which may be applied in any of the devices described herein.Thus a directional backlight 20 as shown in FIG. 25A provides a narrowcone 450, whereas a non-directional backlight 20 as shown in FIG. 25Bprovides a wide angular distribution cone 452 of light output rays.

FIG. 26 is a schematic graph illustrating variation with luminance withlateral viewing angle for various different backlight arrangements. Thegraph of FIG. 26 may be a cross section through the polar field-of-viewprofiles described herein.

A Lambertian backlight has a luminance profile 846 that is independentof viewing angle.

A typical wide angle backlight has a roll-off at higher angles such thatthe full width half maximum 866 of relative luminance may be greaterthan 40°, preferably greater than 60° and most preferably greater than80°. Further the relative luminance 864 at +/−45°, is preferably greaterthan 7.5%, more preferably greater than 10% and most preferably greaterthan 20%.

By way of comparison a directional backlight 20 has a roll-off at higherangles such that the full width half maximum 862 of relative luminancemay be less than 60°, preferably less than 40° and most preferably lessthan 20°. Further the backlight 20 may provide a luminance at polarangles to the normal to the spatial light modulator 48 greater than 45degrees that is at most 33% of the luminance along the normal to thespatial light modulator 48, preferably at most 20% of the luminancealong the normal to the spatial light modulator 48, and most preferablyat most 10% of the luminance along the normal to the spatial lightmodulator 48.

Scatter and diffraction in the spatial light modulator 48 may degradeprivacy mode operation when the switchable retarder 300 is arrangedbetween the input display polariser 210 and additional polariser 318.The luminance at polar angles to the normal to the spatial lightmodulator greater than 45 degrees may be increased in arrangementswherein the switchable retarder 300 is arranged between the outputdisplay polariser 218 and additional polariser 318 in comparison toarrangements wherein the switchable retarder 300 is arranged between theinput display polariser 210 and additional polariser 318.

Advantageously lower off-axis luminance may be achieved for thearrangement of FIG. 1A in comparison to FIG. 2A for the same backlight20.

In an illustrative embodiment of FIG. 1A, the luminance at polar anglesto the normal to the spatial light modulator 48 greater than 45 degreesmay be at most 18% whereas in an illustrative embodiment of FIG. 2A, theluminance at polar angles to the normal to the spatial light modulator48 greater than 45 degrees may be at most 10%. Advantageously theembodiment of FIG. 1A may provide a wider viewing freedom in wide anglemode of operation while achieving similar viewing freedom to theembodiment of FIG. 2A in privacy mode of operation.

Such luminance profiles may be provided by the directional backlights 20described below or may also be provided by wide angle backlights incombination with further additional polariser 318B and passive retarders270 or additional compensated switching liquid crystal retarder 300B.

FIG. 27A is a schematic diagram illustrating in side view a switchabledirectional display apparatus 100 comprising a switchable liquid crystalretarder 300 and backlight 20. The backlight 20 of FIG. 27A may beapplied in any of the devices described herein and which comprises animaging waveguide 1 illuminated by a light source array 15 through aninput end 2. FIG. 27B which is a schematic diagram illustrating in rearperspective view operation of the imaging waveguide 1 of FIG. 27A in anarrow angle mode of operation.

The imaging waveguides 1 is of the type described in U.S. Pat. No.9,519,153, which is herein incorporated by reference in its entirety.The waveguide 1 has an input end 2 extending in a lateral directionalong the waveguide 1. An array of light sources 15 are disposed alongthe input end 2 and input light into the waveguide 1.

The waveguide 1 also has opposed first and second guide surfaces 6, 8extending across the waveguide 1 from the input end 2 to a reflectiveend 4 for guiding light input at the input end 2 forwards and back alongthe waveguide 1. The second guide surface 8 has a plurality of lightextraction features 12 facing the reflective end 4 and arranged todeflect at least some of the light guided back through the waveguide 1from the reflective end 4 from different input positions across theinput end 2 in different directions through the first guide surface 6that are dependent on the input position.

In operation, light rays are directed from light source array 15 throughan input end and are guided between first and second guiding surfaces 6,8 without loss to a reflective end 4. Reflected rays are incident ontofacets 12 and output by reflection as light rays 230 or transmitted aslight rays 232. Transmitted light rays 232 are directed back through thewaveguide 1 by facets 803, 805 of rear reflector 800. Operation of rearreflectors are described further in U.S. Pat. No. 10,054,732, which isherein incorporated by reference in its entirety.

As illustrated in FIG. 27B, optical power of the curved reflective end 4and facets 12 provide an optical window 26 that is transmitted throughthe spatial light modulator 48 and has an axis 197 that is typicallyaligned to the optical axis 199 of the waveguide 1. Similar opticalwindow 26 is provided by transmitted light rays 232 that are reflectedby the rear reflector 800.

FIG. 27C is a schematic graph illustrating field-of-view luminance plotof the output of FIG. 27B when used in a display apparatus with noswitchable liquid crystal retarder.

Thus for off-axis viewing positions observed by snoopers 47 may havereduced luminance, for example between 1% and 3% of the central peakluminance at an elevation of 0 degrees and lateral angle of +/−45degrees. Further reduction of off-axis luminance is achieved by theplural retarders 301, 330 of the present embodiments.

Another type of directional backlight with low off-axis luminance willnow be described.

FIG. 28A is a schematic diagram illustrating in side view a switchabledirectional display apparatus comprising a backlight 20 including aswitchable collimating waveguide 901 and a switchable liquid crystalretarder 300 and additional polariser 318. The backlight 20 of FIG. 28Amay be applied in any of the devices described herein and is arranged asfollows.

The waveguide 901 has an input end 902 extending in a lateral directionalong the waveguide 901. An array of light sources 915 are disposedalong the input end 902 and input light into the waveguide 901. Thewaveguide 901 also has opposed first and second guide surfaces 906, 908extending across the waveguide 901 from the input end 902 to areflective end 904 for guiding light input at the input end 902 forwardsand back along the waveguide 901. In operation, light is guided betweenthe first and second guiding surface 906, 908.

The first guiding surface 906 may be provided with a lenticularstructure 904 comprising a plurality of elongate lenticular elements 905and the second guiding surface 908 may be provided with prismaticstructures 912 which are inclined and act as light extraction features.The plurality of elongate lenticular elements 905 of the lenticularstructure 904 and the plurality of inclined light extraction featuresdeflect input light guided through the waveguide 901 to exit through thefirst guide surface 906.

A rear reflector 903 that may be a planar reflector is provided todirect light that is transmitted through the surface 908 back throughthe waveguide 901.

Output light rays that are incident on both the prismatic structures 912and lenticular elements 905 of the lenticular structure 904 are outputat angles close to grazing incidence to the surface 906. A prismaticturning film 926 comprising facets 927 is arranged to redirect outputlight rays 234 by total internal reflection through the spatial lightmodulator 48 and compensated switchable liquid crystal retarder 300.

FIG. 28B is a schematic diagram illustrating in top view output of thecollimating waveguide 901. Prismatic structures 912 are arranged toprovide light at angles of incidence onto the lenticular structure 904that are below the critical angle and thus may escape. On incidence atthe edges of a lenticular surface, the inclination of the surfaceprovides a light deflection for escaping rays and provides a collimatingeffect. Light ray 234 may be provided by light rays 188 a-c and lightrays 189 a-c, with incidence on locations 185 of the lenticularstructure 904 of the collimated waveguide 901.

FIG. 28C is a schematic graph illustrating an iso-luminancefield-of-view polar plot for the display apparatus of FIG. 28A. Thus anarrow output light cone may be provided, with size determined by thestructures of the structures 904, 912 and the turning film 926.

Advantageously in regions in which snoopers may be located with lateralangles of 45 degrees or greater for example, the luminance of outputfrom the display is small, typically less than 2%. It would be desirableto achieve further reduction of output luminance. Such further reductionis provided by the compensated switchable liquid crystal retarder 300and additional polariser 318 as illustrated in FIG. 28A. Advantageouslya high performance privacy display with low off-axis luminance may beprovided over a wide field of view.

Directional backlights such as the types described in FIG. 27A and FIG.28A together with the plural retarders 301, 330 of the presentembodiments may achieve off-axis luminance of less than 1.5%, preferablyless than 0.75% and most preferably less than 0.5% may be achieved fortypical snooper 47 locations. Further, high on-axis luminance anduniformity may be provided for the primary user 45. Advantageously ahigh performance privacy display with low off-axis luminance may beprovided over a wide field of view, that may be switched to a wide anglemode by means of control of the switchable retarder 301 by means ofcontrol system 352 illustrated in FIG. 1A.

The operation of retarder layers between parallel polarisers foroff-axis illumination will now be described further. In the variousdevices described above, retarders are arranged between a pair ofpolarisers (typically the additional polariser 318 and one of the inputpolariser 210 and output polariser 218) in various differentconfigurations. In each case, the retarders are configured so that theynot affect the luminance of light passing through the pair of polarisersand the plural retarders along an axis along a normal to the plane ofthe retarders but they do reduce the luminance of light passing throughthe pair of polarisers and the plural retarders along an axis inclinedto a normal to the plane of the retarders, at least in one of theswitchable states of the compensated switchable retarder 300. There willnow be given a description of this effect in more detail, the principlesof which may be applied in general to all of the devices describedabove.

FIG. 29A is a schematic diagram illustrating in perspective viewillumination of a retarder layer by off-axis light. Correction retarder630 may comprise birefringent material, represented by refractive indexellipsoid 632 with optical axis direction 634 at 0 degrees to thex-axis, and have a thickness 631. Normal light rays 636 propagate sothat the path length in the material is the same as the thickness 631.Light rays 637 are in the y-z plane have an increased path length;however the birefringence of the material is substantially the same asthe rays 636. By way of comparison light rays 638 that are in the x-zplane have an increased path length in the birefringent material andfurther the birefringence is different to the normal ray 636.

The retardance of the retarder 630 is thus dependent on the angle ofincidence of the respective ray, and also the plane of incidence, thatis rays 638 in the x-z will have a retardance different from the normalrays 636 and the rays 637 in the y-z plane.

The interaction of polarized light with the retarder 630 will now bedescribed. To distinguish from the first and second polarizationcomponents during operation in a directional backlight 101, thefollowing explanation will refer to third and fourth polarizationcomponents.

FIG. 29B is a schematic diagram illustrating in perspective viewillumination of a retarder layer by off-axis light of a third linearpolarization state at 90 degrees to the x-axis and FIG. 29C is aschematic diagram illustrating in perspective view illumination of aretarder layer by off-axis light of a fourth linear polarization stateat 0 degrees to the x-axis. In such arrangements, the incident linearpolarization states are aligned to the optical axes of the birefringentmaterial, represented by ellipse 632. Consequently, no phase differencebetween the third and fourth orthogonal polarization components isprovided, and there is no resultant change of the polarization state ofthe linearly polarized input for each ray 636, 637, 638. Thus, theretarder 630 introduces no phase shift to polarisation components oflight passed by the polariser on the input side of the retarder 630along an axis along a normal to the plane of the retarder 630.Accordingly, the retarder 630 does not affect the luminance of lightpassing through the retarder 630 and polarisers (not shown) on each sideof the retarder 630. Although FIGS. 29A-C relate specifically to theretarder 630 that is passive, a similar effect is achieved by aswitchable liquid crystal retarder and by plural retarders in thedevices described above.

FIG. 29D is a schematic diagram illustrating in perspective viewillumination of a retarder 630 layer by off-axis light of a linearpolarization state at 45 degrees. The linear polarization state may beresolved into third and fourth polarization components that arerespectively orthogonal and parallel to optical axis 634 direction. Theretarder thickness 631 and material retardance represented by refractiveindex ellipsoid 632 may provide a net effect of relatively shifting thephase of the third and fourth polarization components incident thereonin a normal direction represented by ray 636 by half a wavelength, for adesign wavelength. The design wavelength may for example be in the rangeof 500 to 550 nm.

At the design wavelength and for light propagating normally along ray636 then the output polarization may be rotated by 90 degrees to alinear polarization state 640 at −45 degrees. Light propagating alongray 637 may see a phase difference that is similar but not identical tothe phase difference along ray 637 due to the change in thickness, andthus an elliptical polarization state 639 may be output which may have amajor axis similar to the linear polarization axis of the output lightfor ray 636.

By way of contrast, the phase difference for the incident linearpolarization state along ray 638 may be significantly different, inparticular a lower phase difference may be provided. Such phasedifference may provide an output polarization state 644 that issubstantially circular at a given inclination angle 642. Thus, theretarder 630 introduces a phase shift to polarisation components oflight passed by the polariser on the input side of the retarder 630along an axis corresponding to ray 638 that is inclined to a normal tothe plane of the retarder 630. Although FIG. 29D relates to the retarder630 that is passive, a similar effect is achieved by a switchable liquidcrystal retarder, and in the plural retarders described above, in aswitchable state of the switchable liquid crystal retarder correspondingto the privacy mode.

To illustrate the off-axis behaviour of retarder stacks, the angularluminance control of C-plates 308A, 308B between an additional polariser318 and output display polariser 218 will now be described for variousoff-axis illumination arrangements with reference to the operation of aC-plate 560 between the parallel polarisers 500, 210.

FIG. 30A is a schematic diagram illustrating in perspective viewillumination of a C-plate layer by off-axis polarised light with apositive elevation. Incident linear polarisation component 704 isincident onto the birefringent material 632 of the retarder 560 that isa C-plate with optical axis direction 507 that is perpendicular to theplane of the retarder 560. Polarisation component 704 sees no net phasedifference on transmission through the liquid crystal molecule and sothe output polarisation component is the same as component 704. Thus amaximum transmission is seen through the polariser 210. Thus theretarder comprises a retarder 560 having an optical axis 561perpendicular to the plane of the retarder 560, that is the x-y plane.The retarder 560 having an optical axis perpendicular to the plane ofthe retarder comprises a C-plate.

FIG. 30B is a schematic diagram illustrating in perspective viewillumination of a C-plate layer by off-axis polarised light with anegative lateral angle. As with the arrangement of FIG. 30A,polarisation state 704 sees no net phase difference and is transmittedwith maximum luminance. Thus, the retarder 560 introduces no phase shiftto polarisation components of light passed by the polariser on the inputside of the retarder 560 along an axis along a normal to the plane ofthe retarder 560. Accordingly, the retarder 560 does not affect theluminance of light passing through the retarder 560 and polarisers (notshown) on each side of the retarder 560. Although FIGS. 29A-C relatespecifically to the retarder 560 that is passive, a similar effect isachieved by a switchable liquid crystal retarder and by plural retardersin the devices described above.

FIG. 30C is a schematic diagram illustrating in perspective viewillumination of a C-plate layer by off-axis polarised light with apositive elevation and negative lateral angle. In comparison to thearrangement of FIGS. 30A-B, the polarisation state 704 resolves ontoeigenstates 703, 705 with respect to the birefringent material 632providing a net phase difference on transmission through the retarder560. The resultant elliptical polarisation component 656 is transmittedthrough polariser 210 with reduced luminance in comparison to the raysillustrated in FIGS. 30A-B.

FIG. 30D is a schematic diagram illustrating in perspective viewillumination of a C-plate layer by off-axis polarised light with apositive elevation and positive lateral angle. In a similar manner toFIG. 30C, the polarisation component 704 is resolved into eigenstates703, 705 that undergo a net phase difference, and ellipticalpolarisation component 660 is provided, which after transmission throughthe polariser reduces the luminance of the respective off-axis ray.Thus, the retarder 560 introduces a phase shift to polarisationcomponents of light passed by the polariser on the input side of theretarder 560 along an axis that is inclined to a normal to the plane ofthe retarder 560. Although FIG. 29D relates to the retarder 560 that ispassive, a similar effect is achieved by a switchable liquid crystalretarder, and in the plural retarders described above, in a switchablestate of the switchable liquid crystal retarder corresponding to theprivacy mode

FIG. 30E is a schematic graph illustrating the variation of outputtransmission with polar direction for transmitted light rays in FIGS.30A-D. Thus, the C-plate may provide luminance reduction in polarquadrants. In combination with switchable guest-host liquid crystalretarder 301 described elsewhere herein, (i) removal of luminancereduction of the C-plate may be provided in a first wide angle state ofoperation (ii) extended polar region for luminance reduction may beachieved in a second privacy state of operation.

To illustrate the off-axis behaviour of retarder stacks, the angularluminance control of crossed A-plates 308A, 308B between an additionalpolariser 318 and output display polariser 218 which is a linearpolariser, will now be described for various off-axis illuminationarrangements.

FIG. 31A is a schematic diagram illustrating in perspective viewillumination of crossed A-plate retarder layers by off-axis polarisedlight with a positive elevation. Linear polariser 218 with electricvector transmission direction 219 is used to provide a linearpolarisation state 704 that is parallel to the lateral direction ontofirst A-plate 308A of the crossed A-plates 308A, 308B. The optical axisdirection 309A is inclined at +45 degrees to the lateral direction. Theretardance of the retarder 308A for the off-axis angle θ₁ in thepositive elevation direction provides a resultant polarisation component650 that is generally elliptical on output. Polarisation component 650is incident onto the second A-plate 308B of the crossed A-plates 308A,308B that has an optical axis direction 309B that is orthogonal to theoptical axis direction 309A of the first A-plate 308A. In the plane ofincidence of FIG. 31A, the retardance of the second A-plate 308B for theoff-axis angle θ₁ is equal and opposite to the retardance of the firstA-plate 308A. Thus a net zero retardation is provided for the incidentpolarisation component 704 and the output polarisation component is thesame as the input polarisation component 704.

The output polarisation component is aligned to the electric vectortransmission direction of the additional polariser 318, and thus istransmitted efficiently. Advantageously substantially no losses areprovided for light rays that have zero lateral angle angular componentso that full transmission efficiency is achieved.

FIG. 31B is a schematic diagram illustrating in perspective viewillumination of crossed A-plate retarder layers by off-axis polarisedlight with a negative lateral angle. Thus input polarisation componentis converted by the first A-plate 308A to an intermediate polarisationcomponent 652 that is generally an elliptical polarisation state. Thesecond A-plate 308B again provides an equal and opposite retardation tothe first A-plate so that the output polarisation component is the sameas the input polarisation component 704 and light is efficientlytransmitted through the polariser 318.

Thus the retarder comprises a pair of retarders 308A, 308B which haveoptical axes in the plane of the retarders 308A, 308B that are crossed,that is the x-y plane in the present embodiments. The pair of retarders308A, 308B have optical axes 309A, 309B that each extend at 45° withrespect to an electric vector transmission direction that is parallel tothe electric vector transmission of the polariser 318.

Advantageously substantially no losses are provided for light rays thathave zero elevation angular component so that full transmissionefficiency is achieved.

FIG. 31C is a schematic diagram illustrating in perspective viewillumination of crossed A-plate retarder layers by off-axis polarisedlight with a positive elevation and negative lateral angle. Polarisationcomponent 704 is converted to an elliptical polarisation component 654by first A-plate 308A. A resultant elliptical component 656 is outputfrom the second A-plate 308B. Elliptical component 656 is analysed byinput polariser 318 with reduced luminance in comparison to the inputluminance of the first polarisation component 704.

FIG. 31D is a schematic diagram illustrating in perspective viewillumination of crossed A-plate retarder layers by off-axis polarisedlight with a positive elevation and positive lateral angle. Polarisationcomponents 658 and 660 are provided by first and second A-plates 308A,308B as net retardance of first and second retarders does not providecompensation.

Thus luminance is reduced for light rays that have non-zero lateralangle and non-zero elevation components. Advantageously display privacycan be increased for snoopers that are arranged in viewing quadrantswhile luminous efficiency for primary display users is not substantiallyreduced.

FIG. 31E is a schematic graph illustrating the variation of outputtransmission with polar direction for transmitted light rays in FIGS.31A-D. In comparison to the arrangement of FIG. 30E, the area ofluminance reduction is increased for off-axis viewing. However, theswitchable guest-host liquid crystal retarder 301 may provide reduceduniformity in comparison to the C-plate arrangements for off-axisviewing in the first wide mode state of operation.

As may be used herein, the terms “substantially” and “approximately”provide an industry-accepted tolerance for its corresponding term and/orrelativity between items. Such an industry-accepted tolerance rangesfrom zero percent to ten percent and corresponds to, but is not limitedto, component values, angles, et cetera. Such relativity between itemsranges between approximately zero percent to ten percent.

While various embodiments in accordance with the principles disclosedherein have been described above, it should be understood that they havebeen presented by way of example only, and not limitation. Thus, thebreadth and scope of this disclosure should not be limited by any of theabove-described exemplary embodiments, but should be defined only inaccordance with any claims and their equivalents issuing from thisdisclosure. Furthermore, the above advantages and features are providedin described embodiments, but shall not limit the application of suchissued claims to processes and structures accomplishing any or all ofthe above advantages.

Additionally, the section headings herein are provided for consistencywith the suggestions under 37 CFR 1.77 or otherwise to provideorganizational cues. These headings shall not limit or characterize theembodiment(s) set out in any claims that may issue from this disclosure.Specifically and by way of example, although the headings refer to a“Technical Field,” the claims should not be limited by the languagechosen under this heading to describe the so-called field. Further, adescription of a technology in the “Background” is not to be construedas an admission that certain technology is prior art to anyembodiment(s) in this disclosure. Neither is the “Summary” to beconsidered as a characterization of the embodiment(s) set forth inissued claims. Furthermore, any reference in this disclosure to“invention” in the singular should not be used to argue that there isonly a single point of novelty in this disclosure. Multiple embodimentsmay be set forth according to the limitations of the multiple claimsissuing from this disclosure, and such claims accordingly define theembodiment(s), and their equivalents, that are protected thereby. In allinstances, the scope of such claims shall be considered on their ownmerits in light of this disclosure, but should not be constrained by theheadings set forth herein.

1. A display device comprising: a spatial light modulator comprising a layer of addressable pixels; a display polariser arranged on a side of the spatial light modulator, the display polariser being a linear polariser; a guest-host liquid crystal retarder comprising a liquid crystal layer comprising a guest material and a host material, wherein the guest material is an anisotropic material and the host material is a liquid crystal material, the guest-host liquid crystal retarder being arranged on the same side of the spatial light modulator as the display polariser with the display polariser arranged between the guest-host liquid crystal retarder and the spatial light modulator; and at least one passive retarder arranged between the display polariser and the guest-host liquid crystal retarder; wherein the optical axis of the guest-host liquid crystal retarder has an alignment component perpendicular to the plane of the guest-host liquid crystal retarder in at least a state of the host material.
 2. A display device according to claim 1, wherein the anisotropic material is an anisotropic absorber.
 3. A display device according to claim 2, wherein the anisotropic material is a dichroic dye or a pleochroic dye.
 4. A display device according to claim 1, wherein the anisotropic material comprises anisotropic metallic nanomaterial.
 5. A display device according to claim 4, wherein the anisotropic metallic nanomaterial has a transparent electrically insulating surface layer.
 6. A display device according to claim 1, wherein the volume of the guest material is less than 3%.
 7. A display device according to claim 1, wherein the weight of the guest material is less than 3%.
 8. A display device according to claim 1, wherein the guest material comprises a positive dichroic material or a positive pleochroic material and the optical axis of the guest-host liquid crystal retarder has an alignment component in the plane of the guest-host liquid crystal retarder that is orthogonal to the electric vector transmission direction of the display polariser.
 9. A display device according to claim 1, wherein the on-axis extinction coefficient of the guest-host liquid crystal retarder in at least one mode of operation is at least 60%.
 10. (canceled)
 11. A display device according to claim 1, wherein the guest material comprises liquid crystal material that is cured.
 12. A display device according to claim 1, further comprising an additional polariser that is a linear polariser and is arranged on the same side of the spatial light modulator as the display polariser with the guest-host liquid crystal retarder arranged between the display polariser and the additional polariser.
 13. A display device according to claim 12, wherein the display polariser and the additional polariser have electric vector transmission directions that are parallel.
 14. A display device according to claim 12, further comprising at least one passive retarder arranged between the guest-host liquid crystal retarder and the additional polariser.
 15. A display device according to claim 1, wherein the guest-host liquid crystal retarder is a switchable liquid crystal retarder further comprising transparent electrodes arranged to apply a voltage capable of switching host material between at least two states, in one of which states the optical axis of the guest-host liquid crystal retarder has an alignment component perpendicular to the plane of the guest-host liquid crystal retarder.
 16. A display device according claim 15, further comprising a control system arranged to control the voltage applied across the electrodes of the at least one switchable liquid crystal retarder.
 17. A display device according to claim 1, further comprising a backlight arranged to output light, wherein the spatial light modulator is a transmissive spatial light modulator arranged to receive output light from the backlight.
 18. A display device according to claim 17, wherein the backlight provides a luminance at polar angles to the normal to the spatial light modulator greater than 45 degrees that is at most 33% of the luminance along the normal to the spatial light modulator, preferably at most 20% of the luminance along the normal to the spatial light modulator, and most preferably at most 10% of the luminance along the normal to the spatial light modulator.
 19. A display device according to claim 17, wherein the display polariser is an input display polariser arranged on the input side of the spatial light modulator between the backlight and the spatial light modulator, and the guest-host liquid crystal retarder is arranged between the input display polariser and the backlight.
 20. A display device according to claim 1, wherein the display polariser is an output polariser arranged on the output side of the spatial light modulator.
 21. A display device according to claim 1, wherein the spatial light modulator comprises an emissive spatial light modulator arranged to output light and the display polariser is an output display polariser arranged on the output side of the emissive spatial light modulator.
 22. A display device according to claim 1, wherein the at least one passive retarder has its optical axis perpendicular to the plane of the guest-host liquid crystal retarder. 